Three phage-coded functions involved in the expression of bacteriophage Mu immunity

Abstract

Recombinant plasmids harboring different lengths of the immunity end of wild-type Mu DNA were used to study the control of the phage's immunity. Plasmids containing both the smallest ( HindIII.C; 0.7 Md) and the largest ( EcoRI.C; 3.2 Md) fragment confer complete immunity (immunity to superinfection; prevention of induction of a cts prophage). A series of plasmids containing an intermediate-sized fragment ( HindII/ HpaI site are sensitive to superinfection with Mu, but still prevent the induction of a cts prophage. These plasmods can be complemented for expression of immunity by prophage deletions that start at the immunity end, as long as the deletion does not extend beyond gene B. These results indicate that at least three phage genes are involved in the expression of Mu immunity. The repressor, whose synthesis is likely to be basically autoregulated, is able to confer complete immunity. A second gene, mapping to the right of the c gene between 0.7 and 2.6 Md of the left end of Mu DNA, prevents, inhibits, or modulates the expression of gene c. A third gene, which maps between the HindII and the nearby EcoRI site regulates in turn the synthesis or activity of the second gene product. This gene might be identical with the cim function ( M. Giphart-Gassler (1980) Thesis, Univ. of Leiden). By marker rescue experiments and by in vitro protein synthesis directed by the recombinant plasmids, the HindII site has been mapped within gene B.