The Escherichia coli mreB gene encodes an actin-like cytoskeletal protein and is required for rod shape formation of cells and chromosome segregation. Just downstream of mreB, the mreC and mreD genes are located. They are also required for rod shape formation, though their role in chromosome segregation is unclear. lacZ fusion analysis and Northern hybridization showed that the mreB, mreC, and mreD genes formed an operon. Most of the transcripts were expressed as a monocistronic mreB mRNA, and only 1–2% of the transcripts were expressed as a polycistronic mreBCD mRNA. Introduction of a frame-shift mutation in the mreB gene resulted in a significant decrease in the amount of polycistronic mreBCD mRNA but not in that of monocistronic mreB mRNA, suggesting that an attenuation-like regulation was involved in this transcriptional control. Primer extension analysis identified three transcriptional initiation sites. Three possible σ^D-dependent promoter-like sequences were found just upstream of these transcriptional initiation sites. lacZ fusion analysis confirmed that these three promoters contributed to the expression of mreBCD. On the basis of these findings, the essentiality of the mreB gene was confirmed.

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