Summary: Flagellar regions IIIa and IIIb of the and 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, that encodes a cytoplasmic α-amylase. The chromosome between and flagellar region IIIb has now been investigated. The high level of DNA similarity between the and sequences that exists in flagellar region IIIa and in continues initially, with three genes of unknown function; in 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 this region extends for 2 7 kb and in S. typhimurium for 08 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.


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