Two distantly linked clusters of genes on the Salmonella typhimurium phage P22 chromosome are involved in the control of lysogeny and superinfection immunity. One cluster consists of genes cl, ~2, and ~3, which are primarily responsible for the establishment and maintenance of lysogeny. It has been proposed that the second cluster consists of three loci which are responsible for the synthesis and control of an antirepressor substance which overcomes the repression mediated by the c2 gene product. This paper reports the isolation of mutants in a locus designated "ant" having characteristics expected of antirepressor mutants. Evidence is presented that the other loci in this second immunity region, mnt and &rA, control the expression of the ant gene as repressor and promoter/operator, respectively. The interactions of these three loci with each other and with the other immunity region are discussed.
To increase the available set of near-isogenic lines (NILs) for blast-resistance in rice, we have developed a general method for establishing NILs from populations of fixed recombinants that have been used for gene mapping. We demonstrated the application of this method by the selection of lines carrying genes from the rice cultivar Moroberekan. Moroberekan is a West African japonica cultivar that is considered to have durable resistance to rice blast. Multiple genes from Moroberekan conferring complete and partial resistance to blast have previously been mapped using a recombinant inbred (RI) population derived from a cross between Moroberekan and the highly and broadly susceptible indica cultivar CO39. To analyze individual blast-resistance genes, it is desirable to transfer them individually into a susceptible genetic background. This RI population, and the associated data sets on blast reaction and restriction fragment length polymorphism (RFLP) genotypes, were used for selection of lines likely to carry individual blast-resistance genes and a minimum number of chromosomal segments from Moroberekan. Because skewed segregation in the RI population favored CO39 (indica) alleles, resistant lines carrying 8.7-17.5% of Moroberekan alleles (the proportion expected after two or three backcrosses) could be selected. We chose three RI lines carrying different complete resistance genes to blast and two RI lines carrying partial resistance genes to blast as potential parents for the development of NILs. These lines were subjected to genetic analysis, which allowed clarification of some issues that could not be resolved during the initial gene-mapping study.
Mutants of phage P22 which form plaques on a P22 lysogen have been isolated. These virulent mutants have been classified into three groups. (i) VirA mutants arise spontaneously in wild-type stocks and form very small turbid plaques on a P22 lysogen. The single mutation responsible for VirA virulence maps near the mnt locus, one of the immunity regions of phage P22. (ii) VirB mutants do not arise spontaneously and have been isolated only from mutagenized P22 stocks. VirB mutants form small, clear plaques on a P22 lysogen. One of the VirB mutants, virB-3, was analyzed in detail. The virB-3 mutant is comprised of two mutations: K5, which maps within the c2 gene, and Vx, which maps in the region between the c2 and c3 genes. Phages carrying either the K5 or Vx mutation are not virulent in themselves but mutate to VirB virulence at a frequency of 10-5 to 10-6. It is concluded that K5 and Vx are mutations at specific sites which together confer the ability to undergo phage development in the presence of repressor. (iii) VirC mutants are defined by a large clear plaque morphology when plated on a P22 lysogen. VirC mutants are comprised of the determinants of both VirA and VirB virulence.
A replication complex for the vegetative synthesis of the deoxyribonucleic acid (DNA) of the temperate phage P22 previously has been described. This complex is an association of parental phage DNA, most of the newly synthesized phage DNA made during pulses with 3H-thymidine, and other cell constituents, and has a sedimentation rate in neutral sucrose gradients of at least 1,000S. The complex is one of the intermediates, intermediate I, in the synthesis and maturation of phage P22 DNA after infection or induction. Evidence supporting the replicative nature of intermediate I is presented. Phage replication is repressed in lysogenic bacteria. On superinfection of P22 lysogens with nonvirulent phage, little association of the input phage DNA with a rapidly sedimenting fraction is demonstrable. However, after induction with ultraviolet light, the superinfecting parental phage DNA quickly acquires the rapid sedimentation rate characteristic of intermediate I; phage DNA synthesis follows; and progeny phages are produced. Infection with a virulent mutant of P22 produces progeny phages in lysogens. Its DNA associates with intermediate I. In mixed infection with the virulent phage, replication of nonvirulent phage P22 is still repressed, even though the virulent replicates normally. The nonvirulent input DNA does not associate with intermediate I. The repressor of the lysogenic cell prevents replication by interfering with the physical association of template material with intermediate I. A phage function is required for association of phage template with the replication machinery.
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