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Abstract
Bacteriophage λ genome is one of the classical model replicons in studies on the regulation of DNA replication. Moreover, since genes coding for Shiga toxins are located in genomes of lambdoid phages, understanding of mechanisms controlling λ DNA replication may be of bio-medical importance. During lytic development of bacteriophage λ, its genome is replicated according to the [theta] (circle-to-circle) mode early after infection, and then it is switched to the σ (rolling circle) mode. Two mechanisms of regulation of this switch were proposed recently and both suggested a crucial role for directionality of λ DNA replication. Whereas one hypothesis assumed transient impairment of ClpP/ClpX-mediated proteolysis of the λO initiator protein, another suggested a crucial role for transcriptional activation of the oriλ region and factors involved in the control of the p ^sub R^ promoter activity. Here we demonstrate that mutations in clpP and clpX genes had little influence on both directionality of λ DNA replication and appearance of σ replication intermediates. On the other hand, regulators affecting activity of the p ^sub R^ promoter (responsible for initiation of transcription, which activates oriλ) directly or indirectly influenced directionality of λ DNA replication to various extents. Therefore, we conclude that regulation of the efficiency of transcriptional activation of oriλ, rather than transient impairment of the λO proteolysis, is responsible for the control of the switch from [theta] to σ replication, and propose a model for this control.[PUBLICATION ABSTRACT]