Chiggers, Scrub Tyhpus, and Pseudogenes

If you've ever been bitten by tiny red bugs in the garden, you're familiar with members of the Trombiculidae, a family of mites known variously as berry bugs, harvest mites, red bugs, scrub-itch mites, aoutas, or (in the southern U.S.) "chiggers."

In the United States., the garden-variety chigger is basically harmless, but in much of the world this tiny arthropod comes with a very nasty endosymbiont known as Orientia tsutsugamushi, which is a bacterium related to the Rickettsia organisms that cause various tick-borne diseases. Throughout much of the Orient, O. tsutsugamushi infections (from chigger bites) cause scrub typhus, which begins with a rash and fever but can progress to a cough, intestinal distress, swelling of the spleen, abnormal liver chemistry, and ultimately pneumonitis, encephalitis, and/or myocarditis and even death. Treatment with doxycycline, azithromycin, or chloramphenicol is usually successful.

The "harvest mite" (chigger) can carry scrub
typhus, although U.S varieties are typically harmless.

The sequenced genome for O. tsutsugamushi is available, and if you go to this link and click on "Click for features" at the bottom of the Dataset Information box you should be able to open up a table that shows the organism as having 1,182 protein-coding genes (quite a small number), plus an additional 1,994 pseudogenes (quite a huge number, by comparison). The "DNA Seqs" links in the table will let you download the DNA sequences of all the organism's genes and pseudogenes.

This is an extremely unusual situation, in that we're dealing with a bacterium that has more pseudogenes (switched-off, defunct, damaged genes) than regular genes, something that can be said of no other bacterium of which I'm aware. The leprosy bacterium (Mycobacterium leprae) is famed for having approximately 1100 pseudogenes and 1604 "normal" genes. Astonishingly, Orientia tsutsugamushi reverses that ratio, and then some.

We don't know for sure how old Orientia tsutsugamushi's pseudogenes are. A standard rule of thumb in biology is that microbial genomes experience one spontaneous mutation per chromosome per 300 generations. But this doesn't really help us decide how old Orientia's pseudogenes are, since the pseudogenes probably didn't arise one by one, indepedently, through accumulation of random mutations. More than likely, a massive pseudogenization event caused the simultaneous deactivation of a large, unknown number of the organism's genes (of which 1100 survive today as pseudogenes), much the same as has been hypothesized for M. leprae. We have good reason to believe M. leprae's pseudogenes are at least 9 million years old. It seems likely that the pseudogenes in Orientia are also quite old, or at least not terribly new.

To get more perspective on this, I analyzed Orientia's pseudogenes from a couple of perspectives. What I found, first of all, is that the pseudogenes are shorter than their non-pseudo counterparts, averaging 700 bases in length (versus 879 for normal genes). This is similar to the case with M. leprae (where pseudogenes are 795 bases long and normal genes average 1,098). The average shorter gene length for Orientia vis-a-vis M. leprae is consistent with the fact that this is a greatly gene-reduced low-GC (30.5%) endosymbiont, whereas the Mycobacterium family is (in theory) free-living, with higher GC content (57.8% for M. leprae; 65% or more for tuberculosis species).

I've written before about the fact that in most genes, in most organisms, codons tend to begin with a purine base. Therefore I decided to look at purine usage in base one of normal-gene codons versus pseudogene codons (pseudocodons?), finding the following distribution in normal genes:

Purine usage in base one of codons in Orientia tsutsugamushi (N=346,326 codons). No pseudogenes were included in this graph. See the next graph (below) for pseudogenes.

This graph leaves little doubt that most codons begin with a purine (A or G). The median AG1 value is 63.8%. Very few proteins lie to the left of x=0.50, and frankly some of those are probably misannotated as to reading frame.

The situation with pseudogenes is quite a bit different:

Purines in codon base one (AG1) of pseudogenes (N=462,933 codons) in Orientia.

Here we see that purine usage in codon base one is not as strong (median 58.4%), although clearly, plenty of codons still show AG1 above 60%, implying that many pseudogenes are still "in frame" (not frameshifted).

Interestingly, AG1 is not only higher in normal-gene codons than in pseudogene codons, it's also higher in codons associated with proteins of known function than for "hypothetical protein" genes. Only 41.3% of pseudogene codons have AG1 greater than 60%, whereas 66.7% of "hypothetical protein" genes have AG1 > 60% and 84.3% of genes with functional assignments have codon AG1 greater than 60%. This implies that some genes annotated as hypothetical proteins may, in reality, be pseudogenes that are incorrectly annotated. I'll return to that topic some other time.

Are Some Leprosy Pseudogenes Turned On?

Most genomes, whether human or bacterial, contain significant numbers of pseudogenes (that is, genes that are presumed to be inactive due to internal stop codons, frameshift errors, or other serious defects). The usual presumption is that such genes are dormant or dead and thus are not expressed as proteins, since the proteins would be severely truncated or contain nonsense regions, etc.

However, we know that in Mycobacterium leprae (the leprosy bacterium), some 43% of the organism's 1,116 pseudogenes are transcribed. While most of the transcripts are no doubt used in some kind of regulatory capacity, it would be surprising if not a single transcript got translated into protein.

One sign that a protein gene is highly expressed is the presence, upstream of the start codon, of a strong Shine Dalgarno sequence. This is a special sequence of bases that serves as a binding area for 16S ribosomal RNA. The Shine Dalgarno sequence serves to increase the translational efficiency of the genes that have such signatures. (Not all do.) Generally, they are found ahead of high-priority/highly-expressed genes (such as genes for ribosomal proteins). Absence of a SD sequence doesn't mean the gene doesn't get translated. Many genes carry no SD signal.

I created scripts that looked at all 1,116 pseudogenes in M. leprae, to detect the occurrence of Shine Dalgarno sequences in the 20-base-pair region upstream of what would normally be the start codons of said genes. Intriguingly, 31% of pseudogenes carry a length-4 SD sequence (nearly 7 times the number of such sequences expected to occur by chance). By comparison, 50.6% of normal genes in M. leprae carry a length-4 SD sequence.

When I looked for length-5 SD signals, I found that 8.6% of pseudogenes carry such a signal, compared to 26.4% for regular genes. Length-6 signals were found for 33 pseudogenes (3% of the total of 1,116 pseudogenes) versus 176 normal genes (representing 10.9% of 1,604 normal genes). These numbers are about eight times higher than expected to occur by chance.

These numbers are summarized in the table below, where I also show similar findings for genes and pseudogenes of Bordetella pertussis.

Organism	Data Set	Motif Length	Expected	Found	% of Genes
M. leprae	genes (n = 1604)	SD6	5	176	10.9%
		SD5	25	423	26.4%
		SD4	119	812	50.6%
	pseudogenes (n = 1116)	SD6	4	33	3.0%
		SD5	18	96	8.6%
		SD4	83	346	31.0%
B. pertussis	genes (n = 3377)	SD6	9	493	15.0%
		SD5	49	1032	30.6%
		SD4	259	1939	57.4%
	pseudogenes (n = 370)	SD6	0	38	10.20%
		SD5	1	89	24.10%
		SD4	11	194	52.40%

B. pertussis preserves a higher proportion of SD signals in pseudogenes than does M. leprae. This is expected, since most B. pertussis pseudogenes are still "in frame," whereas most (but not all) M. leprae pseudogenes harbor frameshifts.

Length-5-or-longer SD signals occur at about one third the rate in psuedogenes of M. leprae that they do in normal genes of M. leprae, but still much higher than expected by chance. If 43% of pseudogenes are transcribed (as we know they are), and a third of those transcripts have strong enough SD sequences to facilitate translation, it means about 159 pseudogenes in M. leprae could be expected to have expressed protein products. Those products would, of course, be truncated and/or contain nonsense regions. Presumably, many would be marked for proteolysis (either by the tmRNA system or through other mechanisms).

The Pathogen's Playbook

When comparing pathogenic bacteria with non-pathogenic species of the same genus or family, we often find a common pattern. In the pathogen:

• The genome is often reduced in size (particularly in endosymbionts, but also in others).
• The genome is often shifted in the direction of higher A+T content (lower G+C content).
• Many pseudogenes are present.
• Often, the pathogen is a slow-grower in pure culture (if it can be cultured at all).
• The pathogen has special nutritional needs.

An extreme case that illustrates all of these points is Mycobacterium leprae, the leprosy bacterium. It has fewer genes than its cousin, M. tuberculosis (which in turn has fewer genes than non-pathogenic Mycobacteria); its genomic G+C content is 8% lower than most other Mycobacteria; it contains over 1100 pseudogenes; it has a doubling time of two weeks; and it cannot be grown in pure culture (presumably because of fastidious nutritional requirements).

M. tuberculosis can be grown in the laboratory, but it, and its M. avium-group cousins, are very slow growers, taking anywhere from four days to two weeks to develop colonies on solid media.

It seems likely that some pathogens (certainly members of the Mycobacteria, but also the tiny Tenericutes, e.g. Mycoplasma, among many others) have evolved slow growth as a survival strategy. Certainly, organisms that have evolved an intracellular parasitic lifestyle need to be careful not to out-grow the host, if the relationship is to be a long one.

All of the factors listed above suggest a certain scenario, a "pathogen's playbook," if you will, which can be summarized as follows:

1. The organism invades a warm-booded host.
2. Phagocytes (white blood cells) ingest the organism.
3. The phagocytes undergo a respiratory burst, flooding the microbe(s) with peroxides, hypochlorites, nitrous oxide, and other noxious oxidants.
4. The flood of reactive oxygenated species triggers an SOS response in the microbe.
5. The microbe's DNA undergoes massive damage. 
6. Any surviving microbial cells are now pathogenic.

The SOS response is known to trigger mutagenicity. In Mycobacterium, for example, peroxides (as well as UV light) can induce up-regulation of dnaE, an error-prone polymerase. Since Mycobacteria are known to lack a MutS mismatch repair system, SOS-induced errors in DNA replication will almost certainly include uncorrected frameshift errors leading to the creation of pseudogenes. But that's a good thing, if you're a Mycobacterium interested in forming a longterm relationship with a host cell. The loss of certain genes (as long as they're not essential!) will likely slow your metabolism and make you dependent on host nutrients. Truly non-essential pseudogenes will simply be jettisoned over time, reducing the footprint of the remaining genome. Any pseudogenes that survive will likely have done so because they're now playing an essential gene-silencing role.

Let's expand on that last part. Take the dnaE gene, for example. M leprae has two copies of this gene, only one of which is functional. Suppose both copies were functional at the time of the massive pseudogenization event that converted so many of M. leprae's genes to pseudogenes 9 to 20 million years ago. After the pseudogenization event (probably a phagocytic respiratory burst), one copy of dnaE became a pseudogene. But continued transcription of the pseudogene in the forward direction means the pseudo-mRNA competes with the "normal" dnaE transcript for ribosomal attention. Transcription of the antisense strand of the disabled gene would, of course, create a messenger RNA product that could silence the normal transcript by doublestranded interaction. Either way, once the pseudogenization event is over, dnaE expression is attenuated—as it should be, once pathogenicity has been established.

Is it realistic to think M. leprae transcribes antisense strands of its pseudogenes? Given that E. coli has been found to contain ~1000 antisense transcripts, and given that we know M. leprae transcribes many of its pseudogenes, I think the answer has to be yes.

So the pattern is: infection, respiratory burst, massive mutation, silencing of many genes, and (oh by the way) creation of many brand-new gene products, some of them no doubt quite toxic to the host, as the result of gene truncation and pseudogene expression.

Are dead genes still alive in the leprosy bacillus?

The genome of the leprosy bacterium (Mycobacterium leprae) stands as a remarkable example of DNA in an apparent state of massive, wholesale breakdown. Of the organism's 2720 genes, only 1604 appear to be functional, while 1116 are pseudogenes, which is to say genes that have been "turned off" and left for dead.

Genes can become pseudogenes in any number of ways, including loss of a start codon, loss of promoter regions (or degraded Shine Dalgarno signals), random insertions and deletions, mutations that cause spurious stop codons, and so on. Once a gene gets "turned off," assuming loss of the gene in question isn't fatal, the gene typically undergoes a period of degradation (leading to its eventual loss from the genome), but that's not exactly what we see in the leprosy bacterium. When leprosy germs from medieval skeletons were sampled and their genomes sequenced, researchers found that pseudogenes in M. leprae haven't changed very much in the past thousand years or so. Not only does M. leprae tend to hold onto its pseudogenes, it actively transcribes upwards of 40% of them. Probably not all of the transcripts result in expressed proteins (many lack a start codon!), but some no doubt do get translated into proteins. Let's put it this way: It would be extremely unusual for an organism to conserve this many pseudogenes if none of them was doing anything useful.

This view of a segment of the two genomes shows how a region of around 80,000 base pairs in M. tuberculosis maps to a similar 68,000-base-pair region of M. leprae. Notice that in the lowermost panel (representing M. leprae), many genes are shown as shrunken silver segments instead of fat green cylinders. The smaller grey/silver segments are pseudogenes. Click to enlarge.

To get a better idea of what's going on here, I downloaded the DNA sequences of M. leprae's 1604 "normal" genes as well as the 1116 pseudogenes. In analyzing the codons for these genes, I looked for signs of genes that were still in the normal reading frame. One way to detect this is by measuring the purine content at the various base positions in a gene's codons. In a typical protein-coding gene, around 60% of codons begin with A or G (adenine or guanine). This positional bias will, of course, be lost in a gene that has undergone frameshift mutations. Among M. leprae's 1116 pseudogenes, I found 269 in which codons showed an average AG1 percentage (A+G content, codon base one) of 55% or more. These are pseudogenes that appear to still be mostly "in frame."

Things get a lot more interesting where putative membrane proteins are concerned. In a previous post, I showed that in some genes, the second codon base is pyrimidine-rich (i.e., predominantly C or T: cytosine or thymine); these genes encode proteins with a high percentage of nonpolar amino acids. Bottom line, if a gene's codons are mostly T or C in the second position, that gene most likely encodes a membrane-associated protein. (See my previous post for some data.) This is true for all organisms (viruses, cells) and organellar genes, too, by the way, not just M. leprae. It's a generic feature of the genetic code.

When I segregated M. leprae pseudogenes according to whether or not the second codon base was (on average) less than, or more than, 40% purines, I stumbled onto something quite interesting. I found 51 pseudogenes with AG2 less than 40% (meaning, these are probably membrane-associated proteins). Of those, 32 (or 62%) are still "in frame," with AG1 > 55%. By contrast, the majority (78%) of non-membrane pseudogenes (AG2 > 40%) appear to be turned off, with an average AG1 of 51%.

Long story short: Most non-membrane-associated pseudogenes are out-of-frame (and likely dead), whereas 62% of putative membrane-associated pseudogenes appear to be in-frame, and therefore could still be functional (or at least, undead).

In looking at stop codons, I found that of the pseudogenes that still had stop codons, the average distance to the first stop codon is only 149 bases (whereas the average pseudogene length is 795 bases). Pseudogenes for putative membrane-associated proteins were shorter overall (as membrane proteins often are; 495 bases instead of 795), but the average distance to the first stop codon was 190 bases, significantly longer than for the other pseudogenes. This suggests some of them are still alive.

By now you're probably wondering how the heck a pseudogene can be of any possible use whatsoever when it contains a premature stop codon. The thing we need to ask, though, is why M. leprae tolerates (indeed conserves) so many pseudogenes in the first place. Could it be that the organism has adapted a frameshift-tolerant translation apparatus? Maybe some of the stop codons aren't really stop codons.

We know that a wide variety of organisms (not just viruses, where this phenomenon was first discovered, but bacteria and eukaryotes) have evolved special signals to tell ribosomes to shift in and out of frame by plus or minus one. (See "A Gripping Tale of Ribosomal Frameshifting: Extragenic Suppressors of Frameshift Mutations Spotlight P-Site Realignment," Atkins and Björk, Microbiol. Mol. Biol. Rev. 2009.) Certain tRNAs participate in "quadruplet codon" decoding, making it possible for special frameshift signals to work. The signals usually involve 7-base-long "slippery heptamer" sequences, such as CCCTGAC, right where a stop codon (TGA) appears. In other words, when a stop codon appears inside a slippery heptamer, it's not really a stop codon. Depending on the kinds (and amounts) of tRNAs "on duty," it can be a frameshift signal.

When I looked for CCCTGAC in M. leprae's pseudogenes, I found 16 in-frame occurrences of the sequence in 1116 pseudogenes. (Only 7 occurrences of the hexamer CCCTGA were found, in frame, in M. leprae's "normal" genes.) While this doesn't prove that M. leprae is up to any unusual translation tricks, it's a tantalizing result. Also bear in mind, if M. leprae is indeed up to some unusual tricks, it may very well be using frameshift signals other than (or in addition to) CCCTGAC. The fact that Mycobacterium species lack a MutS/MutL mismatch repair system means M. leprae may have adapted different ways of coping with "slippery repeats."

Further work will be needed to confirm whether M. leprae indeed translates some of its pseudogenes into proteins. The 32 "high likelihood" pseudogenes that, according to my analysis, might still encode functional (or at least expressed) membrane-associated proteins are shown in the table below. Leave a comment if you have additional thoughts.

M. leprae pseudogenes that have codons with overall AG1 > 55% and AG2 < 40%:

Pseudogene	Possible product
MLBr00146	hypothetical protein
MLBr00189	hypothetical protein
MLBr00278	conserved hypothetical protein
MLBr00341	hypothetical protein
MLBr00460	hypothetical protein
MLBr00478	hypothetical protein
MLBr00738	PstA component of phosphate uptake
MLBr00836	hypothetical protein
MLBr00846	ABC transporter
MLBr01054	possible PPE-family protein
MLBr01156	hypothetical protein
MLBr01237	possible cytochrome P450
MLBr01238	probable cytochrome P450
MLBr01400	possible membrane protein
MLBr01414	PGRS-family protein
MLBr01474	hypothetical protein
MLBr01527	dihydrodipicolinate reductase
MLBr01673	conserved hypothetical protein
MLBr01792	probable Na+/H+ exchanger
MLBr01968	PE family protein
MLBr02003	probable ketoacyl reductase
MLBr02101	conserved hypothetical protein
MLBr02150	molybdopterin converting factor subunit 1
MLBr02190	PstA component of phosphate uptake
MLBr02216	dihydrolipoamide dehydrogenase
MLBr02363	19 kDa antigenic lipoprotein
MLBr02477	PE protein
MLBr02484	transcriptional regulator (LysR family)
MLBr02533	PE-family protein
MLBr02656	conserved hypothetical protein
MLBr02674	possible membrane protein

Why do so many codons begin with a purine?

With the advent of sites like genomevolution.org (where you can download genomes, create synteny graphs, run BLAST searches, and do all sorts of desktop bioinformatics), it's ridiculously easy for someone interested in comparative genomics to . . . well, compare genomes, for one thing. And if you look at enough gene sequences, a couple of things pop out.

One thing that pops out is that most codons, in most genes, begin with a purine (namely A or G: adenine or guanine). Also, codons typically show the greatest GC swing in base number three. These trends can be seen in the chart below, where I show average base composition (by codon position) for three well-studied organisms. For clarity, base-one purines are shown in bold and base-three G and C are shown highlighted in yellow.

Organism	Codon base	A	G	C	T
S. griseus	1	0.166	0.434	0.287	0.112
	2	0.224	0.219	0.295	0.261
	3	0.037	0.394	0.530	0.038
E. coli	1	0.256	0.343	0.238	0.161
	2	0.291	0.181	0.222	0.304
	3	0.186	0.285	0.261	0.265
C. botulinum	1	0.395	0.299	0.094	0.210
	2	0.374	0.136	0.161	0.328
	3	0.442	0.108	0.064	0.383

Streptomyces griseus is a common soil bacterium that happens to have very high genomic G+C content (72.1% overall, although you can see that in base three of codons the G+C content is more like 92%).

E. coli represents a middle-of-the-road organism in terms of G+C content (50.8% overall), while our ugly friend Clostridium botulinum (the soil organism that can ruin your whole day if it finds its way into a can of tomatoes) has very low genomic G+C content (around 28%).

Even though these organisms differ greatly in G+C content, they all illustrate the (universal) trend toward usage of purines (A or G) in the first position of a codon. Something like 59% to 69% of the time (depending on the organism), codons look like Pnn, where P is a purine base and 'n' represents any base. This is true for viruses as well as cellular genomes.

This pattern is so universal, one wonders why it exists. I think a credible, parsimonious explanation is that when protein-coding genes look like PnnPnnPnn... (etc.) it makes for a crisp reading frame. It's easy to see that a +1 frameshift results in a repeating nPn pattern and a +2 frameshift results in repeats of nnP. These are easily distinguished from Pnn.

There are benefits for a PnnPnnPnn... reading frame. In a previous post, I showed that when most of a gene's codons have a pyrimidine in base two, the resulting protein gets shipped to the cell membrane. (This is a simple consequence of the fact that codons with a pyrimidine in position two tend to code for hydrophobic, lipid-soluble amino acids.) Because a +1 reading-frame shift produces repeats of nPn, the Pnn "default" pattern means that +1 frameshifted gene products, if they occur, won't get shipped to the cell membrane. This is an extremely important outcome, because membrane proteins are, in general, highly transcribed and under strong selective pressure. In addition to specifying antigenic properties and determining phage resistance, membrane proteins make up proton pumps, secretion systems, symporters, kinases, flagellar components, and many other kinds of proteins. They determine the cell's "interface" to the world. They also maintain cell osmolarity and membrane redox potential. Messing with membrane proteins is bound to be risky. Much better to keep frameshifted nonsense proteins away from the membrane.

Fairly strong support for this notion (that Pnn codons provide a crisp reading frame) comes from studies of naturally occurring frameshift signals in DNA. We now know that in many organisms, certain "slippery" DNA signals (usually heptamers, like CCCTGAC) instruct the ribosome to change reading frames. (See, for example, "A Gripping Tale of Ribosomal Frameshifting: Extragenic Suppressors of Frameshift Mutations Spotlight P-Site Realignment," Atkins and Björk, Microbiol. Mol. Biol. Rev. 2009. Also, for fun, be sure to check out some of the papers on quadruplet decoding, which leaves room for alien life forms with 200 amino acids instead of 20.) The "slippery heptamer" frameshift signals that have thus far been identified tend to contain runs of pyrimidines.

Also tending to support the "Pnn = crisp reading frame" notion is the fact that stop codons (TGA, TAA, TAG) look like pPP (where 'p' is a pyrimidine and 'P' is a purine). Again, a crisp distinction.

As for why purines were chosen (and not pyrimidines) to begin the Xxx pattern, again I think a fairly parsimonious answer is available: ATP and GTP are the most abundant nucleoside-triphosphates in vivo. These are the energy sources for nucleic-acid and protein synthesis, respectively.

A prediction: If we run into an alien life form (in the oceans of Europa, say) and it turns out to be the case that UTP (instead of ATP) is the "universal energy molecule" in that life form's cells, then that life form's codons will probably begin with U and form Unn triplets (or Unnn quadruplets, perhaps) a higher-than-average percentage of the time.