THE PCNB GENE OF ESCHERICHIA-COLI, WHICH IS REQUIRED FOR COLE1 COPY NUMBER MAINTENANCE, IS DISPENSABLE

The pcnB gene product of Escherichia coli is required for copy number maintenance of plasmids related to ColE1 and also for that of the IncF II plasmid R1. Because PcnB is similar to the tRNA-binding protein tRN A nucleotidyltransferase, we have suggested that the protein would be required only for processes in which an RNA is a prominent regulatory component. This appears to be so; strains deleted for pcnB, although d efective in ColE1 and R1 plasmid maintenance, maintain the iteron-regu lated plasmids F and P1 normally. We also find that strains deleted fo r pcnB grow normally, demonstrating that PcnB has no essential cellula r role under the conditions tested and suggesting that regulation by a ntisense RNAs similar to RNAI has no critical role in any essential ho st process. We confirm by immunological tests that PcnB is likely to b e the commercially available enzyme poly(A) polymerase.