ESCHERICHIA-COLI GENES EXPRESSED PREFERENTIALLY IN AN AQUATIC ENVIRONMENT

Enteric bacteria are frequently found in aquatic environments, where t hey may pose a risk to human health. Although bacterial survival and p ersistence in such habitats has been studied extensively, there is alm ost no information about bacterial adaptation to these conditions at t he level of changes in gene expression. As a first exploration of this field, we have carried out a screen designed to identify Escherichia coli genes that show increased expression in an aquatic environment. T he screen was performed by subtractive hybridization on a genomic libr ary and led to the identification of several RNA species more abundant in cells inoculated in this medium than in stationary-phase cultures after growth in rich medium. The genes identified include specific tRN A operons and a gene of unknown function, gapC, with similarities to g lyceraldehyde-3-phosphate dehydrogenases. E. coli K-12 strains appear to have accumulated mutations in gapC, which may impede its translatio n, whereas natural isolates have an intact gapC gene. Sequence compari son of gapC with related genes suggests its acquisition by horizontal gene transfer from Gram-positive bacteria.