Evidence against phenotypic mixing between bacteriophage T4 wild type and T4v minus.

In a recent publication Shames et al. (1973) concluded that the UV-specific T4 endonuclease (a repair enzyme coded for by the gene v of wild-type T4) is a component of extracellular phage, which is injected into the host cell and can perform an early repair step without requiring gene expression. This notion is, however, not supported by results presented in this paper. Lysates obtained from mixed multiple infection of Escherichia coli cells with T4v(1) (-) and T4v(+) (or T4v(2) (-) and T4v(+)) failed to show the expected phenotypic mixing, i.e., incorporation of UV endonuclease into capsids of v(-) phages resulting in recognizable repair. The fraction of v(+) and v(-) particles in such lysates was determined by single-plaque analysis before and after irradiation with a UV dose at which virtually all survivors are particles having undergone repair. Even though our mixed infection conditions were most favorable for the possible occurrence of phenotypic mixing, none out of several hundred individual plaques from survivors were found to be genotypically v(-), whereas 30 were expected in the case that phenotypically mixed v(-) particles were repaired like T4v(+). Our failure to observe phenotypic mixing suggests that the data by Shames et al. reflect intracellular synthesis of endonuclease after phage infection.