GENETIC EXCLUSION IN BACTERIOPHAGE-T4 (EXONUCLEASES).

Abstract

Genetic exclusion in phage T4 is the prime responsibility of the imm and sp genes. The map region containing imm does not allow sufficient bps to encode for proteins the size reported for the imm gp. After assaying 30 mutants of the genes adjacent to imm, I found 7 in gene 42 that were defective in the imm phenotype. Upon reverting amNG411(42), the mutant most defective exclusion, for its gene 42 phenotype the exclusion phenotype also changed. When assayed in UGA suppressor hosts, imm+ phage showed a decreased exclusion ability indicating that an opal codon was involved in production of the functional imm gp. I concluded that imm and gene 42 overlap in an out-of-phase orientation with the involvement of an opal readthrough. This overlap has implications in the genetic regulation of this region. This region of T4 also encodes several other genes important in phage intra- and interspecific competition. They are B-gt, 42 and sp. Using recombinant DNA techniques, I precisely located the sp gene to a region between 21.647 and 22.014 kbp on the T4 restriction map and determined its molecular weight as approximately 15 kDa. This same region of T4 was purported to contain gene 40. Complementation and marker rescue experiments with sp+ plasmids indicated that genes sp and 40 are the same. Gene 40 mutants also were found to be defective in sp function. Genes sp and 40 were redesignated gene sp/40 thus linking an early expressing gene with the morphogenic pathway of prohead assembly. Functionally, host enzymes exo III and exo V were found as participants in gp imm mediated exclusion. Presumably gp imm alters the pilot protein of the superinfecting DNA thus exposing it. Gp sp functions by an anti-lysozyme action. But the pleiotrophic effects of sp/40 are best explained by a temperature induced conformational rearrangement hypothesis. This work links molecular genetics to the ecological concept of competition and provides insights into the function and the evolutionary significance of the competition cluster genes. The competition cluster encodes fundamental adaptive strategies found universally in nature.