Mutants of bacteriophage λ which do not require the cIII gene for efficient lysogenization

Abstract

The λ cIII gene is ordinarily required for the high rate, establishment mode of repressor synthesis and efficient lysogenization by phage λ We have isolated phage mutants (called can) which no longer require cIII for efficient lysogenization by selecting turbid plaque-forming pseudorevertants of phages carrying a deletion of cIII. Three different kinds of can mutations have been characterized: can10, which is located in the cro gene; can15, which appears to be identical to cin1 and which is located in the left part of the y region; and can1, which is located in the right part of they region, among the clear cy mutations. Can10 and the previously identified cro − mutation, cro27, both improve lysogenization frequency in the absence of cIII function, possibly because of overproduction of cII protein. In addition, lysogenization by λ cIII + phages carrying these mutations is more efficient than by λ + at low multiplicity of infection. The cro27 mutation also improves lysogenization by cII − phages, suggesting that Cro may prevent premature synthesis of repressor in the maintenance mode. Can1 is dominant to can + and partially relieves the lysogenization defect of λ cIII − in a cap − cya − host. The ability of λ cIII − cI − can1 to stimulate lysogenization by λ cIII − cI + indicates that can1 does not require an adjacent functional cI gene for its action and that can1 affects the activity of a diffusible product, presumably cII protein. This proposal is supported by a cis-trans test showing that can1 requires an adjacent functional cII gene. Although can1 does not increase the frequency of lysogenization in the absence of cII function, it does increase lysogenization frequency when cII activity is present but limiting. These observations suggest that can1 increases the level or activity of cII protein. Based on the ability of can1 and can10, to bypass the need for cIII, we propose that the role of cIII protein in stimulating repressor synthesis is indirect; it acts by increasing activity of the cII protein.