Question

Practice Question:
P22 is a virus very similar to λ. However, it gives rise to virulent mutants at a much higher frequency than λ (1 in 100 vs. 1 in 1000). The reason for this difference is due to a protein produced by P22 called anti-repressor which, when expressed, complexes with and inactivates the P22 phage repressor protein. In light of this information, suggest an explanation for the higher rate of P22 virulent mutations.

Answer 1

P22 phage is a temperate phage of Salmonella typhimurium. It can undergo two types of life cycle- a lytic cycle and a lysogenic cycle. In the lytic cycle, the phage will replicate and form progeny phages which are released when the host cell is lysed. In lysogenic cycle, the prophage will integrate into host chromosome and will replicate within the host chromosome. There is an inhibition to superinfection of the prophage containing host cell by other P22. Induction of lytic cycle in prophage can occur when there is exposure to UV light or antibiotics mitomycin and streptonigrin.

The repressor protein is a protein that inhibits the lytic cycle in lysogenic phage. In P22 phages, the c2 gene expresses the repressor protein. Repressor protein binds to left and right operator sites (OL and OR) on the genes that are required for lytic cycle. Hence, expression of lytic cycle genes will not occur and the phage will remain as a prophage in the host chromosome.

The virulent mutants, formed by exposure of P22 to mutagenic agents or have been mutated, will express an anti-repressor protein. This anti-repressor protein will bind to the repressor protein encoded by C2 gene. As a result, repressor protein is inactivated. Hence, c2 protein cannot bind to the OL and OR. As there is no repressor bound to the operator sites, RNA polymerase can bind to promoter sites and induce transcription of target genes. Hence, the phages will enter the lytic cycle. Hence, virulent mutants are formed at higher rates with P22 phages than lambda phage.