Intergenic cis-trans position effects in bacteriophage T4.

PSTEIN et al. (1963) have defined the “genes” of T4 by means of compleE mentation tests between the members of many pairs of amber and temperature-sensitive mutations. In tests involving at least one amber mutant, the results are clear cut-one observes either no complementation or complementation sufficient to produce a yield of phage comparable to that obtained in infections by wild-type T4 (EDGAR, ~ N H A R D T , and EPSTEIN 1964; EPSTEIN and EDGAR, personal communication; STAHL and MURRAY 1966). However, we chanced to observe (STAHL and MURRAY 1966) a few cases of rather poor complementation in certain control experiments between T4 amber mutants which were considered to be in different genes. For two of the more striking cases we constructed the double amber type in order to conduct a proper cis-trans test (LEWIS 1951, and see BENZER 1955). In both cases the phage yields from cells mixedly infected by wild-type and double amber particles were greater than the yields from cells mixedly infected by each of the single amber types. Either of the two nontrivial standard explanations seemed plausible to us-(1) The product of one or both of the genes involved is unstable except in combination with the other; unless the genes are translated in the immediate neighborhood of each other, one or both of the products decays partially before combining. (2) The translation and/or the transcription of one or the other of the two complementing wild-type genes is being partially depressed by the amber mutation cis to it; i.e., one or the other of the two amber mutants is a “polar” mutant (see NEWTON et al. 1965). In this paper we give details of two cases of cis-trans position effect with T4 amber mutants as well as experiments which demonstrate that the effect is better explained by alternative (2). Our methodology is simple and generalizable. If operons with defined limits exist in T4, our method should permit their identification as well as define direction in which each is polarized.