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Publisher Summary This chapter discusses bacteriophages as biochemical and genetic systems. The experimental evidence discussed in this chapter leads to the conclusion that two complementary activities take place in the reproduction of bacteriophage,. The first is the replication of the genetic material to ensure genetic continuity of the virus. The second is the specification by the genome of a multiplicity of functions which are necessary for successful passage of the phage through its growth cycle. Both of these aspects of bacteriophage growth pose related problems about molecular mechanisms: one is ultimately concerned with the mechanism of replication of DNA molecules, the other with the mechanism of determination by DNA molecules of the structural details of proteins. The central problem is to determine the relation between the nucleotide sequence of the DNA of a cistron and the amino acid sequence of the protein that it specifies. The best way to attack this problem is to isolate a protein specified by a cistron for which genetic fine structure analysis is possible.

Mitochondrial mutations conferring resistance to specific antibiotics have been isolated in the yeast Saccharomyces cevevisiae. These mutations are located in the mitochondrial DNA (mtDNA) molecule (Nagley and Linnane, 1972; Faye et al., 1973; Deutsch et al., 1974). Mitochondrial recombination in yeast is interesting to consider in a symposium on mechanisms in recombination for several reasons: (1) mitochondrial genetics is a population genetics similar in certain aspects to bacteriophage genetics, (2) the elementary acts of recombination are of the nonreciprocal type, and (3) the occurrence of repetitive sequences in mtDNA of ρ- mutants offers a tool to study recombination in segments of repeated genes. Recently, extensive studies on multifactorial mitochondrial crosses have been published (Kleese et al., 1972; Suda and Uchida, 1972; Avner et al., 1973; Wolf et al., 1973; Howell et al., 1973; Rank, 1973 and Netter et al., 1974), using a standard cross procedure described by Coen et al. (1970), in which a random sample of a population issued from many zygotes after some 20 cell divisions is analyzed.

RECENTLY, Tessman1,2 and Sinsheimer3 suggested that the deoxyribonucleic acid of ΦX 174 is one-stranded in contrast to the double-stranded deoxyribonucleic acid of other phages. From this point of view it seemed interesting to investigate genetic recombination in this phage. Tessman4 achieved recombination in bacteriophage S 13 by stimulation with ultra-violet irradiation of free virus particles. (S 13 is serologically related to ΦX 174 and similar in other properties.) The frequency of recombinants found after ultra-violet irradiation was of the order of 0.1 per cent. In other work5 without irradiation, no recombination could be detected.

The modification of restricted phage T1 (rT1) to the unrestricted form (uT1), and the analogous modification of phage P2 (rP2 to uP2) can be brought about by the presence in the same nonlysogenic host cell, of the vegetative phase of the phage P1. In P1-lysogenic host cells, joint infection with P1 and rT1 increases only slightly the yield of uT1 as compared to infection with rT1 alone. It is postulated that the rare event that causes P1-lysogenic hosts to become yielders when infected with rT1 or rP2, is prophage activation and that a subsequent interaction between vegetative P1 and the restricted phage causes some of the latter to become modified.