We obtained a mutation designated cpeB2 that maps in or very close to rpoB (the gene encoding b -subunit of RNA polymerase) with the following phenotypes : (i) conditional lethality on low osmolarity medium at 420C; (ii) moderate increase in proU expression at low osmolarity; and (iii) lethality of all cells in stationary phase following transformation with the ColE1-like plasmid vector pACYC184 (1). We are at present undertaking a detailed genetic and molecular characterization of cpeB2 including isolation of suppressors of the pACYC184-lethality phenotype. One class of suppressor mutations maps to the hns gene (1). Preliminary data suggest that an association exists between the onset of lethality of pACYC184 transformants and a massive increase in the plasmid copy number in the cells. It is therefore expected that these studies will shed light on the roles of various chromosomal genes on regulation of copy number of the ColE1-like plasmids.