Lysogenic control of temperate phage 16-3 of Rhizobium meliloti 41 is governed by two distinct regions

Summary In addition to the regulator gene C of temperate phage 16-3 of Rhizobium meliloti 41, a second repressor function, called immune X, was identified and cloned into the low copy number cosmid vector pLAFR1. Both repressor functions are necessary to establish complete immunity against superinfecting non-virulent 16-3 strains, but either of the two alone decreases the efficiency of plating (e.o.p.) dramatically. It was shown that the primary target of gene product immune X was the avirT operator locus. The coding region of immune X was localized to the left arm of the phage genome, inside the EcoRI L and H fragments. This region maps about 14 kb from cistron C and had been thought to be genetically silent.