IMMUNITY DETERMINANT OF PHAGE-PLASMID P4 IS A SHORT PROCESSED RNA

In the phage-plasmid P4, both lysogenic and lytic functions are coded by the same operon. Early after infection the whole operon is transcri bed from the constitutive promoter P-LE In the lysogenic condition tra nscription from P-LE terminates prematurely and only the immunity func tions, which are proximal to the promoter, are thus expressed. Fragmen ts of the P4 immunity region were cloned in an expression vector. A DN A fragment as short as 91 bp was sufficient, when transcribed, to expr ess P4 immunity and to complement P4 immunity deficient mutants. This fragment, like prophage P4, produced a 69 nt long RNA (CI RNA). A shor ter P4 fragment neither expressed immunity nor synthesized the CI RNA. Thus the CI RNA is the P4 trans-acting immunity factor. The 5' end of the CI RNA, mapped by primer extension, does not correspond to the tr anscription initiation point, thus suggesting that the CI RNA is produ ced by processing of the primary transcript. In an RNase P mutant host the processing of the 5' end and the production of a functional CI RN A were impaired. The requirement of RNase P for the correct processing of CI appears to be related to the predicted secondary structure of t he precursor CI RNA. A region (seqB) within the CI RNA shows complemen tarity with two cis-acting sequences (seqA and seqC) required for P4 i mmunity, suggesting that transcription termination may be caused by pa iring of the CI RNA with the complementary target sequences on the nas cent transcript.