The S and U genes of bacteriophage Mu are located in the invertible G segment of Mu DNA

Abstract

An F′ plasmid containing only the β and G segments of bacteriophage Mu DNA in a β-G-β structure was isolated as a LacZ − segregant of an F′ lac plasmid containing Mu prophages inserted in the lacI and lacY genes. The segregant arose by homologous recombination between the similarly oriented G regions located in oppositely oriented Mu prophages whose α segments were directed toward the lacZ gene. Electron microscopic observation of single-stranded plasmid DNA from the segregant revealed the expected β-G-β stem and loop structure in which the double-stranded stem was the same length as, β and the single-stranded loop was the same length as G. Marker rescue experiments with Mu amber mutants defective in each of the known essential Mu genes showed that strains carrying this F′-β-G-β plasmid contained the wild-type alleles for S and U mutations but not for mutations in other genes. more detailed mapping of the R S U region by deletion mapping in Mu prophages and λpMu transducing phages produced two gene orders, R S U and R U S, thus indicating that the S and U genes were located in the invertible G segment rather than in β. Confirmation of this conclusion was obtained from results of DNA heteroduplexing experiments which showed that deletions ending within gene U end physically within G segment DNA. The location of essential genes S and U within G and the location and extent of G segment DNA which is nonessential for growth ( L. T. Chow, R. Kahmann, and D. Kamp, 1977, J. Mol. Biol., 113, 591–609), taken together, reveal that the order of genes in viable Mu phage with G in the G(+) orientation is R S U.