Evidence for effect of random genetic drift on G+C content after lateral transfer of fucose pathway genes to Escherichia coli K-12

The cps cluster of Escherichia coli K-12 comprises genes involved insynthesis of capsular polysaccharide colanic acid. Part of the E. coli K-12cps region has been cloned and sequenced and compared to its Salmonellaenterica LT2 counterpart. The cps genes from the two organisms arehomologous; in the case of the LT2 genes, with G+C content of 0.61 andcodons characteristic of high G+C species, it seems clear that they havebeen acquired relatively recently by lateral transfer from a high G+Cspecies. The K-12 form of these cps genes is closely related to those ofLT2 so must derive from the same high G+C species, but it appears to havetransferred much earlier such that random genetic drift has brought P3 (thecorrected G+C content of codon base 3) down from 0.77 to 0.64, more thanhalfway to the E. coli average of 0.57. We estimate, using an equationdeveloped by Sueoka, that the lateral transfer to E. coli took placeapproximately 45 million years ago. This is the first report we are awareof demonstrating the expected adjustment of P3 after lateral transferbetween species with different G+C content DNA.