Involvement of the invertible G segment in bacteriophage mu tail fiber biosynthesis

The orientation [G(+) or G(-)] of the invertible G segment of bacteriophage Mu DNA determines the host range specificity of the phage particles. In this study the hypothesis that the G segment genes are involved in synthesis of Mu tail fibers has been tested. Serum blocking power (SBP) assays demonstrated that among Mu late gene mutants only those defective in genes S or U encoded by the G segment were defective in G(+) SBP and that they lacked the same antigens. Electron microscopy of lysates produced by inversion-defective gin mutants (isolated by their inability to complement a hin inversion-defective mutant of the Salmonella phase variation segment) showed that G(+) phages with amber mutations in S or U made tail-fiberless particles with contracted tail sheaths. Inversion of G to the G(-) orientation or suppression of the amber mutations restored the normal phage particle morphology. These experiments demonstrate that genes S and U are required for Mu G(+) tail fiber biosynthesis and/or attachment.