CHARACTERIZATION OF THE PRIMARY IMMUNITY REGION OF THE ESCHERICHIA-COLI LINEAR PLASMID PROPHAGE N-15

N15 is the only bacteriophage of Escherichia coli known to lysogenize as a linear plasmid, Clear-plaque mutations lie in at least two region s of the 46-kb genome, We have cloned, sequenced, and characterized th e primary immunity region, immB. This region contains a gene, cB, whos e product shows homology to lambdoid phage repressors. The cB3 mutatio n confers thermoinducibility on N15 lysogens, consistent with CB being the primary repressor of N15, Downstream of cB lies the locus of N15 plasmid replication, Upstream of cB lies an operon predicted to encode two products: one homologous to the late repressor of P22 (Cro), the other homologous to the late antiterminator of phi 82 (Q), The Q-like protein is essential for phage development, We show that CB protein re gulates the expression of genes that flank the cB gene by binding to D NA at symmetric 16-hp sites, Three sites are clustered upstream of cB and overlap a predicted promoter of the cro and e-like genes as well a s two predicted promoters of cB itself. Two sites downstream of cB ove rlap a predicted promoter of a plasmid replication gene, repA, consist ent with the higher copy number of the mutant, N15cB3. The lender regi on of repA contains terminators in both orientations and a putative pr omoter, The organization of these regulatory elements suggests that N1 5 plasmid replication is controlled not only by CB but also by an anti sense RNA and by a balance betweem termination and antitermination.