The N gene product of phage λ controls the expression of most other λ -functions (Chapter 2). This control appears to be exerted at the level of RNA transcription, since in the absence of active N product λ transcription is drastically reduced. The N gene product acts in two ways to permit the vegetative growth of λ : (1) N stimulates transcription of the xc II OP operon (see Dove, 1968), and (2) N is necessary for Q synthesis (Dambly et al., 1968), a λ function necessary for transcription of late λ genes (Skalka et al., 1967). Dependence on N can be eliminated by the presence of two mutations: (1) a promoter mutation ( c 17) which permits high levels of N independent transcription of the c II OP operon (Packman and Sly, 1968), and (2) byp, a mutation located between P and Q, which permits synthesis of Q in the absence of N (Butler and Echols, 1970; Hopkins, 1970). In addition to its role in vegetative growth, N is also needed for transcription of the region to the left of immunity which codes for λ functions unessential for vegetative growth, but essential for lysogeny and recombination (Chapter 6). Although studies with λ mutants have been useful in defining the role of N in λ growth, they have yielded little information as to the mechanism of action of this function. A host involvement in N activity has been postulated by Roberts (1969) and studies with Escherichia coli-Salmonella typhosa hybrids suggested that this may be the case. A hybrid...