The Circular Linkage Map of Bacteriophage T2h.

HE purpose of this paper is to report extensive linkage data obtained with host range (ht) mutants which confirm the circular nature of the T2 coliphage genetic map. The genetic map of T2 was once considered to be two independent linkage groups until STREISINGER and BRUCE (1960) demonstrated its unity. Although the experimental evidence was not published until 1964 ( STREISINGER, EDGAR and DENHARDT 1964), STREISINGER, in 1961, first suggested a circular genetic map as the explanation of the results of certain crosses with T4 mutants. Extensive studies (EPSTEIN et al., 1963; EDGAR, DENHARDT, and EPSTEIN 1964; and EDGAR and LIELAUSIS 1964) using many temperature sensitive ( t s ) and amber mutants ( a m ) showed that in T4 no two ts and am sites are sufficiently widely separated to be termed unlinked. FOSS and STAHL (1963), using rescue experiments, showed that the order of four distantly linked markers in T4 could be arranged on a circular map. The best interpretation of all these data is a circular linkage map. BAYLOR and SILVER 1961, showed that certain ht mutants were mutations of genes which determine some of the proteins of the mature phage particle. Their experiments were designed to distinguish whether the gene directly determined a structure o r modified the product of another gene. To extend this work on mechanism of gene control, we applied the same tests to other parts of the genome which contained heat sensitive ht mutants. We feel the results of these experiments make it likely that ht mutants occur only in genes which directly control the structural proteins of the phage. The ht mutants control the adsorption of the phage to the resistant cell B/2 and affect the heat stability of the mature phage as well. Each of these properties exhibits phenotypic mixing and may be characteristic of any part of the phage external protein. The different mutants must be located in genes which determine different structures of the phage because they are distributed over two thirds of the linkage map.