Bacteriophage phi 29 early protein p17 is conditionally required for the first rounds of viral DNA replication

The gene 17 of the Bacillus subtilis phage phi 29 is known to be involved i n the viral DNA replication in vivo. Tn this paper, we show that the presen ce of protein p17 is required when phage infection occurs at a low multipli city of infection (moi), which is probably the natural condition for infect ion, but is dispensable at a high moi. Gene 17 has been cloned in an Escher ichia coli expression vector and protein p17 purified. A stimulatory effect of protein p17 was demonstrated under in vitro conditions required to ampl ify phi 29 DNA, starting with a low amount of input DNA. We propose that p1 7, which is synthesized early after infection, is required at the very begi nning of the phage amplification, conditions in which a low number of viral DNA molecules enter the host cell, possibly to recruit the limiting amount of initiation factors at the replication origins. Once the infection proce ss is established and the other replication proteins reach optimal concentr ation, p17 becomes dispensable. (C) 1998 Elsevier Science B.V. All rights r eserved.