ROLE OF GPFI PROTEIN IN DNA PACKAGING BY BACTERIOPHAGE-LAMBDA

One of the final steps in the assembly of bacteriophage lambda is the excision of a single genome from a concatemeric DNA precursor and inse rtion of this monomer into a preformed capsid. Terminase enzymes are c ommon to all of the double-stranded DNA phages, and in lambda this enz yme is responsible for both excision of a genome monomer from the conc atemer and its insertion into the pro-capsid. We have previously demon strated that the endonuclease activity of lambda terminase (cos-cleava ge) was stoichiometric with enzyme and postulated that this was due to formation of a stable, postcleavage enzyme DNA intermediate (complex I) (Tomka & Catalano, 1993b). Bacteriophage lambda gpFI protein is req uired for efficient assembly of the virus, and current models suggest that this protein increases the rate of pro-capsid binding to complex I. We show here that gpFI markedly stimulates cos-cleavage by lambda t erminase, even in the absence of viral pro-capsids. Importantly, the o bserved increase in nicking activity did not result from an increase i n the I-ate of cos-cleavage, but rather by an increase in turnover by the enzyme. These data suggest that gpFI destabilizes complex I, thus allowing terminase release from cos and catalytic turnover by the enzy me. The implications of these results with respect to terminase assemb ly onto viral DNA, nicking of the duplex, and subsequent translocation during packaging are discussed.