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The invertible G segment in phage Mu DNA controls the host range of the phage. Depending on the orientation of the G segment, two types of phage particles, G(+) and G(-), are produced which recognize different cell surface receptors. The receptor for Mu G(+) was located in the lipopolysaccharide (LPS) of gram-negative bacteria. The analysis of different LPS core types and of mutants that were made resistant to Mu G(+) shows that the primary receptor site on Escherichia coli K-12 lies in the GlcNAc beta 1 . . . 6Glc alpha 1-2Glc alpha 1-part at the outer end of the LPS. Mu shares this receptor site in E. coli K-12 with the unrelated single-stranded DNA phage St-1. Phage D108, which is related to Mu, and phages P1 and P7, which are unrelated to Mu but contain a homologous invertible DNA segment, have different receptor requirements. Since they also bind to terminal glucose in a different configuration, they adsorb to and infect E. coli K-12 strains with an incomplete LPS core.

The leftmost 1590 bp of Mu DNA covering the immunity region have been sequenced. This region encodes the cI repressor, the cII or ner function and the beginning of gene A. An open reading frame extends from position 863 to 342 on the l-strand corresponding to cI protein with a molecular weight of 19212. It is preceded by a sequence resembling a promoter. To the right of the HindIII site an open reading frame extends from position 1099 to 1323 corresponding to cII or ner protein (molecular weight of 8505) followed by the beginning of gene A at position 1328. Between position 863 and 1099 promoters for leftward and rightward transcription and operator-like structures can be recognized in the sequence. The promoter for rightward transcription overlaps with the HindIII site and coincides with a RNA polymerase binding site as demonstrated by electron microscopy.

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