A phage P1 virulent mutation at a new map location

Working with Phage P22

Transduction experiments in Escherichia coli and Salmonella are usually performed with virulent phage variants. A widely used P1 mutant, called P1 vir, carries one or more uncharacterized mutations that prevent formation of lysogens. In the case of P22, by far the most frequently used variant is named P22 HT105/1 int-201 This phage has a high transducing (HT) frequency due to a mutant nuclease with lower specificity for the pac sequence. As a result, ∼50% of the P22 HT phage heads carry random transducing fragments of chromosomal DNA. The int mutation reduces the formation of stable lysogens. The basic steps in handling the P22 HT105/1 int-201 phage and in performing transduction experiments in Salmonella are described here.

The ImmC region of phage P1 codes for a gene whose product promotes lytic growth

The ImmC region of the temperate bacteriophage P1 contains c1, a gene that codes for a repressor of lytic growth. Located in the region upstream of c1 are four open reading frames capable of coding for low-molecular-weight proteins. The efficiency of lysogeny by P1+Cm was found to be reduced by almost 10(5)-fold when the host cells carry this region of ImmC on a multicopy plasmid. The sequences responsible for interfering with lysogen formation were localized to one of the small open reading frames (orf4) within ImmC. Insertions and deletions within orf4 suppress the virulent phenotype of P1virC mutants when introduced into the phage by recombination. These virC-suppressed mutant phage were found to be incapable of lytic growth unless the product of orf4 is provided in trans. The presence of orf4 was observed to interfere with repression by the c1 protein of ImmC-encoded promoters fused to lacZ. For this reason, we suggest that orf4 corresponds to coi, a gene previously proposed to code for an inactivator of c1-mediated repression.

Characterization of the binding sites of c1 repressor of bacteriophage P1. Evidence for multiple asymmetric sites

The repressor of bacteriophage P1, encoded by the c1 gene, is responsible for maintaining a P1 prophage in the lysogenic state. In this paper we present: (1) the sequence of the rightmost 943 base-pairs of the P1 genetic map that includes the 5'-terminal 224 base-pairs of the c1 gene plus its upstream region; (2) the construction of a plasmid that directs the production of approximately 5% of the cell's protein as P1 repressor; (3) a deletion analysis that establishes the startpoint of P1 repressor translation; (4) filter binding experiments that demonstrate that P1 repressor binds to several regions upstream from the c1 gene; (5) DNase I footprint experiments that directly identify two of the P1 repressor binding sites. Sequences very similar to the identified binding sites occur in at least 11 sites in P1, in most cases near functions known, or likely, to be controlled by repressor. From these sites we have derived the consensus binding site sequence ATTGCTCTAATAAATTT. We suggest that, unlike other phage operators, the P1 repressor binding sites lack rotational symmetry.

Identification of the repressor and repressor bypass (antirepressor) polypeptides of bacteriophage P1 synthesized in infected minicells

P1 infected minicells synthesize approximately 50 phage-encoded polypeptides. Phage expression is temporally controlled, demonstrating phage polypeptides synthesized both early and late after infection. The P1 repressor, gpcl1 (Mr = 33,000), repressor bypass polypeptide, gpreb A (Mr = 27,500) and cistron 10 product, (gp10) (Mr = 64,000), have been identified by infection of minicells with P1 amber mutants. The beta-lactamase gene product (gpbla) carried by the closely related P7 and the chloramphenicol acetyl-transferase gene product (gpcat) carried by P1 Cm (in Tn9) have been demonstrated. Infection of minicells by P1virs or P1c4 mutants results in increased synthesis of gpreb A and a second polypeptide designated gpreb B (Mr = 40,000). The P1vir11 mutation leads to increased synthesis of a small polypeptide (Mr = 3,500) but does not affect the amount of gpc1 synthesized.