Defining cosQ the site required for termination of bacteriophage lambda DNA packaging

Bacteriophage lambda is a double-stranded DNA virus that processes concatem eric DNA into virion chromosomes by cutting at specific recognition sites t ermed cos. A cos is composed of three subsites: cosN, the nicking site; cos B, required for packaging initiation; and cosQ required for termination of chromosome packaging. During packaging termination, nicking of the bottom s trand of cosN depends on cosa suggesting that cosQ is needed to deliver ter minase to the bottom strand of cosN to carry out nicking. In the present wo rk, saturation mutagenesis showed that a 7-bp segment comprises cosQ, A pro posal that cosQ function requires an optimal sequence match between cosQ an d cosNR, the right cosN half-site, was tested by constructing double cosQ m utants; the behavior of the double mutants was inconsistent with the propos al. Substitutions in the 17-bp region between cosQ and cosN resulted in no major defects in chromosome packaging. Insertional mutagenesis indicated th at proper spacing between cosQ and cosN is required. The lethality of integ ral helical insertions eliminated a model in which DNA looping enables cosQ to deliver a gpA protomer for nicking at cosN. The 7 bp of cosQ coincide e xactly with the recognition sequence for the Escherichia coli restriction e ndonuclease, EcoO0109I.