The structure of the phi 80d3 ilv+ Su+7 transducing phage and the origin of its Su+7 tRNA-gene containing fragment

Genetics of bacteriophage phi 80--a review

The genetic maps of bacteriophage lambda and lambdoid phage phi 80 are compared. The gene organization of phi 80 is very similar to that of lambda, as shown by isolation and characterization of many am, ts and c (clear) mutants of the phage. In general, the essential genes located in the same position on the genetic map of the phages lambda and phi 80 fulfill the same functions. These include the gene clusters coding for the head and tail proteins, genes for DNA synthesis, and the genes controlling lysogeny and late gene expression. The specific regulatory features of phi 80 in relation to the N function of lambda are discussed, but they require further clarification. The two phages differ in immunity specificity, host range, conversion property and temperature sensitivity.

Construction of a composite tRNA gene by anticodon loop transplant

By using sites for the restriction nuclease Hpa II, the information for the anticodon stem and loop of an altered Su+2 amber suppressor tRNA (a mutant of tRNAGln) has been transplanted to a specially prepared tRNATrp gene, which lacks it homologous anticodon stem and loop sequence. The resulting tRNA gene was cloned under lac operator-promoter control. The result is a functional, hybrid, amber-suppressor tRNA that can exhibit a moderately high efficiency in translation. It appears less efficient, however, than Su+7 tRNA, the amber suppressor that results from a direct anticodon mutation in tRNATrp. As judged by its suppressor spectrum, which is almost identical to the spectra of Su+2, and Su+7, the recomposed tRNA inserts glutamine at amber sites. This experiment is the prototype of a series of construction that examine the role of the nucleotides in the anticodon region.

Relaxation of stable RNA synthesis by a plasmid-borne locus

The plasmid pMY3, which was constructed so as to express the Su+7 amber suppressor tRNA gene, also relaxes control of stable RNA synthesis in stringent cells. The relaxation is not growth medium or strain-dependent and does not occur in the presence of the vehicle alone. When expression of the effective sequence is diminished, in a lysogen of phi 80d3 ilv+Su+7, the sequence no longer affects RNA synthesis. The relaxation is general, extending to all or almost all tRNA loci, including tRNAs located in the ribosomal spacer regions, and to all ribosomal RNAs. Relaxed plasmid-carrying strains are still able to elevate guanosine tetra- and penta-phosphate levels in response to amino acid starvation, but steady state levels are somewhat diminished. Aminoacyl-tRNA falls to control levels when the plasmid-carrying strain is deprived of amino acid. Therefore, the relaxed strain perceives amino acid starvation, but does not respond normally. These properties define a novel locus which relaxes stringent control.