Invertible DNA determines host specificity of bacteriophage mu.

The function of the invertible G region of bacteriophage Mu is apparently to confer different host specificities on Mu. Two products of genes S and U, situated in the G region are not needed for the infectivity of Mu G(-) particles. In the Mu G(-) phage the S gene product and the 21-K polypeptide, presumably the product of gene U, are missing. Instead, two other polypeptides with different molecular weights are observed.