STUDIES OF MUTATIONS IN T4 CONTROL GENES 33 AND 55

Available mutations in transcriptional control genes 33 and 55 of coliphage T4 have been examined. By complementation analysis and map position, 15 mutants (13 in T4D, 2 in T4B) have been shown to lie in gene 33 and 6 (5 in T4D, 1 in T4B) in gene 55. According to patterns of suppression and recombination, these mutants define three distinct amber sites in gene 33 and also three distinct amber sites in gene 55. All of these mutations are true amber mutations, in apparent contrast to some traditional T4 "amber" mutants which grow in su+  E. coli CR63 but not in su-  E. coli B because of a strain difference other than the su+ determinant. Evidence is presented that, contrary to previous suggestions (Bolle et al. 1968; Pulitzer and Geiduschek 1970), the gene 33 product is absolutely essential for T4 development.