Organization of the Escherichia coli and Salmonella typhimurium chromosomes between flagellar regions IIIa and IIIb, including a large non-coding region.

Flagellar regions IIIa and IIIb of the Escherichia coli and Salmonella typhimurium chromosomes (at 40 min and 42-43 min, respectively) has been shown to be separated by DNA unrelated to flagellar function, with region IIIa being immediately followed by a gene, amyA, that encodes a cytoplasmic alpha-amylase. The chromosome between amyA and flagellar region IIIb has now been investigated. The high level of DNA similarity between the E. coli and S. typhimurium sequences that exists in flagellar region IIIa and in amyA continues initially, with three genes of unknown function; in E. coli, there may be a fourth gene. The remainder of the region, up to the start of flagellar region IIIb, lacks any obvious open reading frames, scores poorly on an algorithm for coding probability, has a high A+T content, and is totally dissimilar in the two species. We conclude that it is non-coding. In E. coli this region extends for 2.7 kb and in S. typhimurium for 0.8 kb. These values are unusually large for prokaryotes, where the non-coding regions between operons are generally quite short. The data, which are discussed in the context of a hypothesized disruption of a contiguous ancestral flagellar region, may give new insight into the organization and evolution of the bacterial chromosome.