Phase-shift and other mutants in the first part of therII B cistron of bacteriophage T4

Abstract

The mutants considered consist of over 200 of the phase-shift type and 16 of the base-substitution type. A set of 61 of these has been mapped with precision and the rest have been located approximately. The results of all the crosses needed to locate the various mutants are listed. The sites of mutation are spread fairly evenly over the genetic map, with only a few appreciable gaps. A sign has been allocated to all the phase-shift mutants except for a group of nine at the extreme left-hand end of the gene. All but two of the base-substitution mutants are ochres or ambers. A complete index is given of all the multiple mutants so far constructed. The phenotype of most of these combinations follows the rules previously proposed. No combination gives the wild or pseudo-wild phenotype which is not expected to do so. The nature of the ‘barriers’ produced in certain pairs of mutants of unlike sign has been explored. Some of these barriers are ochres or ambers, but at least four of them fall into neither of these classes. All of them can be removed by a mutation of the base-substitution type. Certain combinations of double mutants of the type (+ +) have been shown to produce minute plaques on K. Most of these are associated with two of the barriers.