Genetic Map of Bacteriophage T 3 HILDBURG BEIER

Comparative studies with T3 and T7 are of interest because these related phages show several distinctive features which might be helpful for understanding their physiologies of infection. Thus, for instance, simultaneous mixed infection of a host cell by T3 and T7 leads to "mutual exclusion," i.e., each mixedly infected cell generally yields only one type of progeny phage (3). Mutual exclusion is probably at least one of the reasons for the low recombination rates between T3 and T7 (2). These low rates are in contrast to the rather extensive base sequence homologies between these phages (1). As a basis for further comparative genetic analyses of T3 and T7 it seemed indicated to construct a genetic map for phage T3 and to relate it to Studier's (8) detailed genetic map for T7. For this purpose we have further investigated the amber mutants of T3 whose isolation and preliminary characterization has been described by Hausmann and Gomez (4). First, about 200 independently isolated mutants were assigned to complementation groups (genes) by mixed infection of the nonpermissive host in liquid culture, as described previously (5, 9). The mutants could thus be classified into 14 genes. In four instances representatives of more than one spot-test complementation group (4) showed no complementation among themselves when tested in liquid cultures; therefore, they were here assigned to the same gene. Most or all of the mutants in one gene were then crossed in all possible combinations among themselves; the technique was that of Hausmann and Gomez (4). The two mutants separated by the longest map interval within one gene were then selected as gene representatives. These generepresenting mutants were crossed in all combinations. In some cases, to be described in detail elsewhere (H. Beier, Ph.D. thesis, Univ. Freiburg), three-factor crosses with double mutants were also performed. Thus, it was possible to order the mutational sites on a linear map of about 100 map units. In Fig. 1 this map is compared to the T7 map published by Studier (8). The same nomenclature proposed by this author for T7 has been adopted for T3. The assignment of the T3 genes to their homologous counterparts of T7 was done-except for gene 1, whose product does not complement heterologous phage (4, 5)-through heterologous complementation studies with gene-representing mutants of T3 and T7, as described elsewhere (5). In addition to that, in the cases of genes 1, 3, 5, and 17, assays for the corresponding gene products, respectively: RNA polymerase, DNase, DNA polymerase, and serum blocking protein (5, 7) confirmed the assignments. Further evidence for the postulated homologies came (i) from comparing patterns of T3directed DNA synthesis by gene-representing amber mutants during abortive infection with those patterns corresponding to T7 mutants (6), (ii) from the gene sequence on the map, and, especially, (iii) from the analysis of phagecoded proteins by the gel electrophoresisautoradiography method (Issinger and Hausmann, manuscript in preparation). Five genes for which T7 amber mutants are available, have not yet given origin to known mutants of T3 (Fig. 1). It is noteworthy that these are the genes for which relatively few T7 mutants are known (8). Thus, it seems likely that our lack of mutants in the postulated T3 genes is due to chance; in this case a saturating search for further mutants would be expected to be successful. However, it cannot be excluded that mutants in these genes have not been found because they are not essential for T3 growth and thus escaped detection by our isolation procedure. Some bias in our isolation method is suggested by the disproportionately high number of gene 1 mutants, as compared to mutants in other genes (Table 1). Lack of these genes in T3 is unlikely, since Davis and Hyman