The can (previously yadF) gene of Escherichia coli encodes a ␤-class carbonic anhydrase (CA), an enzyme which interconverts CO 2 and bicarbonate.Various essential metabolic processes require either CO 2 or bicarbonate and, although carbon dioxide and bicarbonate spontaneously equilibrate in solution, the low concentration of CO 2 in air and its rapid diffusion from the cell mean that insufficient bicarbonate is spontaneously made in vivo to meet metabolic and biosynthetic needs. We calculate that demand for bicarbonate is 10 3 -to 10 4 -fold greater than would be provided by uncatalyzed intracellular hydration and that enzymatic conversion of CO 2 to bicarbonate is therefore necessary for growth. We find that can expression is ordinarily required for growth in air. It is dispensable if the atmospheric partial pressure of CO 2 is high or during anaerobic growth in a closed vessel at low pH, where copious CO 2 is generated endogenously. CynT, the single E. coli Can paralog, can, when induced with azide, replace Can; also, the ␥-CA from Methanosarcina thermophila can at least partially replace it. Expression studies showed that can transcription does not appear to respond to carbon dioxide concentration or to be autoregulated. However, can expression is influenced by growth rate and the growth cycle; it is expressed best in slow-growing cultures and at higher culture densities. Expression can vary over a 10-fold range during the growth cycle and is also elevated during starvation or heat stress.

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Deletion of ftsK results in the inhibition of cell division, but this inhibition can be reversed by a plasmid carrying only the first ∼17% of ftsK. The division block can be suppressed in most mutants by deletion of dacA, which codes for the d-alanine:d-alanine carboxypeptidase PBP5, or in all mutants by overexpression of ftsN. Overexpression of ftsK inhibits cell division and the formation of FtsZ rings. This division block is not due to the induction of either the SOS or the heat shock regulons.

We report that the genes abc, yaeC, and yaeE comprise metD, an Escherichia coli locus encoding a DLmethionine uptake system. MetD is an ABC transporter with Abc the ATPase, YaeE the permease, and YaeC the likely substrate binding protein. Expression of these genes is regulated by L-methionine and MetJ, a common repressor of the methionine regulon. We propose to rename abc, yaeE, and yaeC as metN, metI, and metQ, respectively.

The replication of ColE1-related plasmids is controlled by an unstable antisense RNA, RNAI, which can interfere with the successful processing of the RNAII primer of replication. We show here that a host protein, PcnB, supports replication by promoting the decay of RNAI. In bacterial strains deleted for PcnB a stable, active form of RNAI, RNAI*, which appears to be identical to the product of 5'-end processing by RNAase E, accumulates. This leads to a reduction in plasmid copy number. We show, using a GST-PcnB fusion protein, that PcnB does not interfere with RNAI/RNAII binding in vitro. The fusion protein, like PcnB, has polyadenylating activity and is able to polyadenylate RNAI (and also another antisense RNA, CopA) in vitro.