Mechanism of transcription of bacteriophage S13. I. Dependence of messengerRNA synthesis on amount and configuration of DNA.

Abstract

The rate of transcription of wild-type bacteriophage S13 increases linearly with time until it reaches a plateau at 10 to 12 minutes after infection. In contrast, a gene A amber mutant, blocked in DNA replication, shows a constant rate of transcription, 4 to 5 times lower than the plateau level of the wild type. The rate of transcription increases with increasing multiplicities of infection of the gene A mutant, until a maximum rate is reached at a multiplicity of 15 to 20 suggesting that the total number of replicative form DNA molecules that can participate in transcription is limited to 15 to 20 per cell. A shift-up of a gene A temperature-sensitive mutant to the non-permissive temperature does not reduce the transcription rate once progeny replicative form DNA synthesis has occurred; thus, neither the gene A protein nor concomitant DNA replication are needed for phage-specific transcription. The level of phage-specific mRNA synthesis depends on the amount of replicative form DNA template in the cell. Parental replicative form DNA alone has a four- to five-fold higher template efficiency for mRNA synthesis than the combination of parental and progeny replicative form DNA that is formed in the presence of an active gene A product. Two mutually compatible explanations for the differential efficiency are: (a) the parental replicative form DNA may be bound to transcription sites that exist in limited numbers, and (b) in the absence of an active gene A product the molecular configuration of the parental replicative form DNA (form I, covalently closed, supercoiled) may make it more efficient. All phage mRNA is found in polysomes. The small amount of mRNA made by the gene A mutant may be translated more efficiently than the mRNA made in excess of that amount by the wild type.