Ner, a Cro-like function of bacteriophage Mu

Abstract

Constitutive expression of three early Mu genes ner, A, and B was obtained by cloning these genes in different combinations downstream of the kanamycin promoter located on the plasmid pACYC177. These Mu genes are involved in bacteriophage Mu transposition ( A and B) and in regulation of Mu transposition ( ner). Strains carrying plasmids that expressed either the ner or ner and A genes were immune to Mu infection although the Mu repressor gene was not present. This pseudoimmunity is due to an almost complete inhibition of early Mu RNA synthesis by the plasmid-encoded Ner product. Evidence is presented that Ner regulates, in addition to transposition functions, synthesis of phage Mu repressor. The ner gene product was identified in minicells as a protein of molecular weight approximately 8000.