Biochemical changes accompanying the partial genetic exclusion of wild-type T2 genes in mixed infections with various mutants of phage T4 have been examined. Total phage yields varied between 14 and 60% of those observed after infections by either wild-type phage alone. DNA synthesis was delayed, but then occurred at rates close to those found with each wild-type phage strain. Among the progeny virus, the frequencies of the excluded wild-type T2 genetic markers ranged between 0.02 and 0.45, and varied with the markers' positions on the genetic map. Measurement of the frequencies of markers among the progeny phage at 15 to 60 min. after infection showed that the weakly excluded T2 genes 1 and 20 increased slightly, while the strongly excluded T2 genes 33 and 42 dropped sharply and then remained constant. The strongly excluded T2 genes 42 and 56 formed low levels of their respective products deoxy-CMP-hydroxymethylase and deoxy-CTP-ase in mixed infections with the corresponding T4 amber mutants in a non-suppressor host. The T2-induced enzymes thymidylate synthetase and lysozyme reached normal levels during infections with the corresponding defective T4 strains. With mixed infections in an endonuclease-I deficient host, [P]-labelled T2 phages yielded 14% of their DNA as acid-soluble products, while no breakdown of [P]-labelled T4 DNA was detected. The possibility is discussed that partial exclusion results from T4-induced nuclease action against T2 chromosomes followed by marker rescue of T2 DNA fragments with intact T4 genomes.


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