EVIDENCE FOR EFFECT OF RANDOM GENETIC DRIFT ON G-COLI K-12(C CONTENT AFTER LATERAL TRANSFER OF FUCOSE PATHWAY GENES TO ESCHERICHIA)

The cps cluster of Escherichia coli K-12 comprises genes in synthesis of capsular polysaccharide colanic acid. Part of the E. coli K-12 cps region has been cloned and sequenced and compared to its Salmonella en terica LT2 counterpart. The cps genes from the two organisms are homol ogous; in the case of the LT2 genes, with G+C content of 0.61 and codo ns characteristic of high G+C species, it seems clear that they have b een acquired relatively recently by lateral transfer from a high G+C s pecies. The K-12 form of these cps genes is closely related to those o f LT2 so must derive from the same high G+C species, but it appears to have transferred much earlier such that random genetic drift has brou ght P-3 (the corrected G+C content of codon base 3) down from 0.77 to 0.64, more than halfway to the E. coli average of 0.57. We estimate, u sing an equation developed by Sueoka, that the lateral transfer to E. coli took place approximately 45 million years ago. This is the first report we are aware of demonstrating the expected adjustment of P-3 af ter lateral transfer between species with different G+C content DNA.