Antisense RNA-dependent transcription termination sites that modulate lysogenic development of satellite phage P4

In the lysogenic state, bacteriophage P4 prevents the expression of its own replication genes, which are encoded in the left operon, through premature transcription termination. The phage factor responsible for efficient termination is a small, untranslated RNA (CI RNA), which acts as an antisense RNA and controls transcription termination by pairing with two complementary sequences (seqA and seqC) located within the leader region of the left operon. A Rho-dependent termination site, t(imm), was previously shown to be involved in the control of P4 replication gene expression. In the present study, by making use of phage Phi R73 as a cloning vector and of suppressor tRNA(Gly) as a reporter gene, we characterized two additional terminators, t(1) and t(4). Although transcription termination at neither site requires the Rho factor, only t(1) has the typical structure of a Rho-independent terminator. t(1) is located between the P-LE promoter and the cI gene, whereas t(4) is located between cI and t(imm). Efficient termination at t(1) requires the CI RNA and the seqA target sequence; in vitro, the CI RNA enhanced termination at t(1) in the absence of any bacterial factor. A P4 mutant, in which the t(1) terminator has been deleted, can still lysogenize both Rho(+) and Rho(-) strains and exhibits increased expression of CI RNA. These data indicate that t(1) and the Rho-dependent t(imm) terminators are not essential for lysogeny. t(1) is involved in CI RNA autoregulation, whereas t(4) appears to be the main terminator necessary to prevent expression of the lytic genes in the lysogenic state.