Genetics of cosQ, the DNA-packaging termination site of phage λ: Local suppressors and methylation effects

Abstract

The cos site of the bacteriophage λ chromosome contains the sites required for DNA processing and packaging during virion assembly. cos is composed of three subsites, cosQ, cosN, and cosB. cosQ is required for the termination of chromosome packaging. Previous studies have shown cosQ mutations to be suppressed in three ways: by a local suppressor within cosQ by an increase in the length of the λ chromosome; and by missense mutations affecting the prohead's portal protein, gpB. In the first study reported here, revertants of a set of cosQ mutants were screened for suppressors, and cis-acting suppressors of cosQ mutations were studied; these included second-site cosQ point mutations, base-pair insertions within cosQ, and an additional genome-lengthening suppressor. The 7-bp-long cosQ, with the sequence 5′-GGGTCCT-3′, coincides exactly with the recognition site for the EcoO1091 restriction/methylation system, which has the consensus sequence 5′-PuGGNCCPy-3′. In a second study, EcoO1091 methylation was found to strongly interfere with the residual cosQ function of leaky cosQ mutants. cis-acting suppressors that overcome methylation-associated defects, including a methylation-dependent suppressor, were also isolated. Models of cosQ suppression are presented.