MECHANISMS OF MUTATION IN BACTERIOPHAGE T4

在大肠杆菌中生长的T4噬菌体是一个祖先模型系统

Bacteriophage T4 growing in Escherichia coli is an ancestral model system for studying mutagenesis and DNA repair. Recent work has produced the following results. (1) Heat produces G:C -> T:A transversions via modification of G residues in nonreplicating T4 DNA. The process is highly variable, appears to depend on trace components of the medium, and may involve a spontaneously activated oxygen species. (2) Spontaneous mutation rates in the T4 rII system are largely independent of both temperature and medium. (3) Growth at pH 9 or mutations in the cya or crp genes strongly increases resistance to UV in E. coli, but not in T4. The photoreactivation and excision-repair systems interact in E. coli but not in T4. However, a T4 mutant defective in replication repair appers to be refractory to photoreactivation. (4) Forward mutation in the T4 system is usually scored by counting r mutants, which arise at five genes. Two of these, rI and rV, remain uncharacterized. The rV gene has been mapped and is being sequenced. (5) Phage RB69 is a distant relative of T4. The RB69 DNA polymerase shares only 61.5% amino acid identify with the T4 DNA polymerase but the RB69 gene can complement a defect in the T4 gene. The fidelity of DNA replication in this configuration is close to normal in vivo.