Björn H. Lindqvist

Mutual derepression in the P2–P4 bacteriophage system☆

Abstract P4 lysogens are derepressed by infection with P2 as is evident from the induction of P4 phage production by P2 infection of P4 lysogens. Mutations interfering with P2 DNA synthesis greatly increase the P4 yield, but DNA replication by the infecting phage does not preclude P4 induction. Of four P2 mutations known to block spontaneous phage production by P2 lysogens, three ( cox-2, cox-3, cox-4 ) also prevent derepression of P4 lysogens by P2. These studies also revealed that, under certain conditions, P4 can complement P2 mutants deficient in gene B function which is required for P2 DNA replication. P2 lysogens are derepressed by infection with P4. This derepression leads to a P2 gene A-dependent P2 prophage replication and, in P2 lysogens mixedly infected with P2 and P4, also to the replication of the DNA of the coinfecting P2. If the derepressing P4 is deficient in gene a function, then derepression of the P2 lysogen will lead to the production of 10 or more P2 phages per cell, provided that P2 prophage excision is enhanced by the P2 nip mutation or is bypassed by infecting the P2 lysogen with P2 as well as with P4. Gene α proficient P4 interferes with the P2 phage production by P2 lysogens; possible causes for this interference are considered, including P4 DNA replication which is known to require gene a function.

Coliphage P2 late control gene ogr: DNA sequence and product identification

The bacteriophage P2 late control gene ogr was cloned and precisely localized by deletion analysis in vitro. The DNA sequence of the ogr gene containing the ogr1 mutation was determined. The sequence translates into a basic protein of a molecular weight of 8300. Plasmids overproducing the ogr gene product were constructed, and the ogr gene product was identified by polyacrylamide gel electrophoresis.

Directed mutagenesis of the bacteriophage P2 ogr gene defines an essential function

The ogr gene of bacteriophage P2 codes for a basic protein of 72 amino acids which is thought to be essential for activation of P2 late gene transcription. However, conditionally lethal mutations in the ogr gene have never been isolated. We have constructed a P2 ogr deletion mutant by in vitro techniques. This deletion phage, P2-del15, grows in a host which provides the ogr gene product in trans from a plasmid but fails to grow in hosts lacking the ogr plasmid. This demonstrates that the ogr gene is essential for P2 lytic growth. The deletion in P2del15 has removed about half of the carboxy-terminal part of the ogr gene. The transcript from this deletion mutant can be distinguished from the wild-type transcript by S1 nuclease protection. The analysis of such transcripts suggests that the ogr gene product may negatively regulate its own transcription.

Control of gene activation of P2 prophage by satellite phage P4.

Abstract In the preceding paper we report that satellite phage P4 can derepress P2 lysogens ( Six and Lindqvist, 1978 ). We now show that derepression leads to the expression of the early as well as the late gene regions of the P2 prophage. In the absence of transactivation, P2 prophage late genes will be expressed only if the early P2 gene A function is available. This was demonstrated with P4 mutants defective in gene α. Such mutants are proficient in derepression but were found to be transactivation deficient, hence restricted to derepression as a means of turning on P2 prophage late genes. Thus two modes of P4-induced late gene expression can be distinguished: transactivation and derepression. Derepression precedes transactivation by about 30 min, but the extra time needed for transactivation appears not to be due to a requirement for early P2 gene products.

Recombination between satellite phage P4 and its helper P2: II. In vitro construction of a helper-independent P4::P2 hybrid phage

Abstract A helper-independent P4::P2 hybrid (Hy19), with the essential gene region of P4 linked to the late genes of P2, has been isolated by in vitro recombination techniques. This hybrid expresses a P4 Sid − phenotype since it makes large heads. The int-C region of P2 is deleted from Hy19 and its DNA replication is independent of the host rep gene, indicating that it depends on the P4 replicon.

The P 2–P 4 Transactivation System

The interactions between bacteriophage P 2 and its satellite phage P 4 comprise a promising model system for studying the control of transcription because the two phage have relatively small and therefore tractable genomes and yet exhibit a surprisingly complex set of effects upon one another. Each phage possesses the ability to drastically alter the expression of the other’s genome.