Bacteriophage lambda DNA packaging in vitro. The involvement of the lambda FI gene product, single-strand DNA, and a...

The FI gene product (gp) of bacteriophage lambda is required during phage head assembly in vivo. Mutations in this gene lead to an accumulation of immature concatemeric lambda DNA and of proheads that appear normal and are competent for DNA packaging in vitro. This phenotype can be taken as evidence of a failure to couple DNA and proheads for packaging/maturation. In contrast to the requirement for gpFI in vivo, the packaging of lambda DNA in vitro occurs efficiently in the complete absence of gpFI. However, if ssDNA is included at the outset of the in vitro packaging reaction, DNA packaging is blocked. This block to packaging is relieved by addition of gpFI. Thus packaging of lambda DNA in vitro can be made dependent of gpFI by the inclusion of ssDNA at the outset of the reaction. Inhibition of DNA packaging by ssDNA appears to be mediated by a lambda b region-directed protein (packaging inhibitor, ben protein) that is present in the crude extracts of cells used to support the early steps of the packaging reaction. Neither ssDNA nor the packaging inhibitor alone has significant inhibitory effect on packaging; both components are required together to effect the inhibition that is relieved by gpFI. The packaging inhibitor was extensively purified and shown to have endonucleolytic activity. Several lines of evidence are presented to support the idea that both the inhibitory and endonucleolytic activities are functions of the same protein. Although gpFI relieves the inhibition imposed by the ben protein in packaging, gpFI fails to block the DNA cleavage activity of the ben protein in the standard endonuclease assay.