Inceptor and origin of DNA replication in lambdoid coliphages II. The λ DNA maximal replication system

In pBR313-lambda dv hydrid plasmids a second system for initiation of DNA replication has been detected in lambdoid replicator DNAs (in the absence of the p0 promoter). The "maximal" (or "maxi") initiation system depends on the origin of replication (ori) sequence, in conjuction with the "inceptor" (ice) element located in the lambdoid cII genes. Only leftward, but not bidirectional, primer RNA synthesis seems to be initiated at ori in its newly defined boundaries, and it appears to be catalysed by dnaG-coded primase. Only if transcriptionally activated, will ori effectively initiate lambda specific, O and P-dependent "maximal" hybrid-plasmid replication. In addition, it will repress a complete lambda "minimal" initiation system in cis, i.e., if present on the same plasmid molecule. This newly discovered repressive activity of the ori system depends on only three factors: an intact left section of ori, the O product, and transcriptional activation of ori (rightward or leftward). A repressed minimal initiation system will regain its activity as soon as a segment carrying either part of the O gene or a promoter for transcriptional activation is delected from such a plasmid which was combining both the "mini" and "maxi" systems of lambda replication.