INVOLVEMENT OF THE HOST INITIATOR FUNCTION DNAA IN THE REPLICATION OFCOLIPHAGE-LAMBDA

We demonstrate that the initiation of coliphage lambda DNA replication is dependent on the host initiator function dnaA, provided that the l ambdoid prophage Rac is absent. Presence of Rac compensated the absenc e of dnaA function, causing initiation of replication. In dnaAts rac() cells at 43 degrees, most of parental phage DNA molecules, after one round of theta replication, switched to a replication with features o f the sigma mode and produced progeny at high yield. Initiation of rep lication of the lambda Pts1 mutant at 43 degrees was blocked by dnaA f unction; however, under dnaA(-)rac(+) conditions all parental phage DN A molecules, after one round of theta replication, switched to the sig ma mode and produced progeny at high yield. Taking into account our re cent finding that transcriptional activation of ori lambda seems to be dnaA-regulated (to be published elsewhere), we suggest that the DnaA- lambda Pts1 incompatibility occurs at the insertion of the ori lambda- bound lambda O-lambda P-DnaB preprimosome between the complementary la mbda DNA strands. The role of Rac and the mechanism of the switch from theta to sigma mode of lambda phage DNA replication are discussed.