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

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.