The transposases! The contest, in case you wonder, was for the most abundant set of genes in the known universe, or at least in the genomic data banks available on Earth. Aziz, Breitbart, and Edwards examined some 10 million genes from 2137 sequenced genomes (47 archaeal, 725 bacterial, 29 eukaryotic and 1336 viral) plus 187 metagenomes, in search of the gene set that is both most abundant and most ubiquitous. (Ubiquitous refers to genes that carry essential functions and are thus needed by every genome. A gene that is highly abundant in some ecosystems but not ubiquitous is likely to play an important habitat-specific role.) The winners, by far, were the genes for transposases. One could squawk that the genomes selected for sequencing to date reflect a bias towards “germs” and other microbes of human interest, and that so far the source environments for metagenomes represent only a small sample of what’s out there. But given the off-scale preponderance of the transposase genes, there is little to make one question their dominance.
Not everybody seems to have transposases; only 1/3 of the viruses in this study have them. But those organisms that do tend to possess multiple copies — close to 40 on average. If you take a set of 2000 randomly sampled genes (the number found in a typical bacterial genome), 22 will encode transposases. They are also more common in free-living bacteria than in obligate pathogens or endosymbionts. The champion transposase-carrier? Possibly the marine cyanobacterium Crocosphaera watsonii, which has more than 400 copies.
So, why transposases? The authors point to their general usefulness to both viruses and cellular organisms in promoting the spread of mobile elements, thus leading to diversification. “Selfish genes” indeed! As the authors write, By securing their own replication and dissemination, transposases guarantee to thrive so long as nucleic acid-based life forms exist. This rather implies that they might imagine non-nucleic acid based life forms exist somewhere. Who says that bioinformaticists are just computational nerds?
Aziz RK, Breitbart M, & Edwards RA (2010). Transposases are the most abundant, most ubiquitous genes in nature. Nucleic acids research PMID: 20215432