Deletion Mapping of the Site of Action of the tof Gene Product

Abstract

The regulation of exonuclease synthesis during the lytic cycle of phage λ is an example of temporal control of gene expression. This enzyme, coded by the phage exo gene (Fig. 1), is synthesized early after infection (Korn and Weissbach, 1963). Synthesis is turned off during the latter part of the phage lytic cycle. However, defective phage bearing a mutation in the x region, which prevents transcription of the r strand of phage λ (Taylor et al., 1967; Kourilsky et al., 1968), do not turn off λ exonuclease synthesis (Radding, 1964; Eisen et al., 1966). The overproduction of exonuclease by x mutants is a recessive phenotype: after induction of bacteria carrying both an x mutant prophage and a wild type λ prophage, exonuclease synthesis is once again turned off during the lytic cycle (Radding, 1964). On the other hand, after induction of bacteria carrying an x mutant prophage and a λimm⁴³⁴ prophage, exonuclease production is not turned off (Pero, 1970). Phage λimm⁴³⁴ is identical to phage λ except for a small portion, 5.5% of the phage genome, that includes the cI gene (Kaiser and Jacob, 1957; Westmoreland et al., 1969). The above findings prompted the following conclusions: (1) there is an additional gene on the x–y–cII–O–P–Q operon which makes a product that can act in trans to turn off exonuelease production, and (2) this gene is different in phage λimm⁴³⁴, and therefore must be located in the leftmost portion of the x–y–cII...