Deletion analysis of two nonessential regions of the T4 genome

Abstract

Mutants of T4 which contain a duplication of the rII region also often contain a compensating deletion of a nonessential segment of the genome. This relationship was used to isolate a series of viable deletion mutations. The deletions were mapped genetically, making use of their effect on plaque morphology, and by electron microscopy of DNA heteroduplexes to T2, a T2 T4 hybrid, and T4 wild type. They were also tested in several laboratories for various phage functions, enzymes, and proteins. Two different overlapping deletions located between genes 32 and 63 and ten located between genes 39 and 56 were obtained. Analysis of the former class indicates that there are at least 1400 base pairs of DNA between genes 32 and 63 which are unaccounted for by presently known genes. Deletions of the latter class affect the production of exonuclease A, a DNA-dependent ATPase, a 5′-phosphatase, and two proteins revealed by polyacrylamide gel electrophoresis. They also affect modification of host RNA polymerase, suppression of unwinding protein deficiency (sud phenotype), and ability to grow on two different Escherichia coli strains. The effect of different deletions on these properties demonstrates that at least seven genes are involved and allows their approximate locations to be determined. The general applicability of the method used for selecting deletions of nonessential DNA is discussed.