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Saccharomyces kluyveri is a heterothallic yeast with two allelic mating types denoted as a-k and α- k by analogy with Saccharomyces cerevisiae and from the work described here. S. kluyveri produces mating pheromones analogous to those of S. cerevisiae , but which appear to have different specificity. S. kluyveri thus differs from S. cerevisiae, Hansenula wingei , and Schizosaccharomyces pombe in that it exhibits both strong constitutive agglutination and mating pheromones. α- k cells produce a pheromone (“α- k -factor”) which causes a-k cells to arrest in the G1 phase of the cell cycle and to undergo a morphological change. After a period of time dependent on the concentration of α- k -factor, cells exposed to the factor resume cell division. α- k -factor has no effect on a-k /α- k diploids or on α- k cells, but at high concentration does induce G1 arrest of S. cerevisiae a cells ( a -c). a-k cells produce a pheromone (“ a-k -factor”) which causes α- k cells to exhibit a morphological change. In addition, a-k cells exhibit the Bar phenotype with respect to α- k -factor. Partially purified preparations of S. cerevisiae α-factor are more active in inducing G1 arrest of a-k cells than of a -c cells. A more purified preparation of α-c-factor is less active against a-k cells than a -c cells, suggesting that an additional factor (KRE, kluyveri response enhancer) may be lost during purification. Attempts to mate S. kluyveri and S. cerevisiae cells by prototroph selection and by cell-to-cell mating have been unsuccessful with all combinations of mating types. Thus, S. cerevisiae and S. kluyveri are incompatible for mating even though their pheromones exhibit some physiological cross-reaction.

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In vitro formation of both bacteriophage T4 internal peptides (II and VII) from preexisting precursor protein was shown to require the product of T4 gene 21. The proteolytic factor was detectable in extracts of cells infected with certain phage mutants blocked in early steps of head assembly but could not be demonstrated in extracts of T4 wild-type infected cells. This finding suggests that the proteolytic factor is inactivated during normal phage assembly. The product of T4 gene 22 appears to be the precursor of peptide VII but not of peptide II.