The bacteriophage Mu gem gene: a positive regulator of the C operon required for normal levels of late transcription.

The gem product of bacteriophage Mu modulates synthesis of various host proteins and alters the host chromosome topology. To elucidate the role of the gem gene in Mu development, we analyzed the behavior of several mutants in this gene. The results, obtained with two Mu gem- phages, show that (1) phage growth is significantly delayed and inhibited, (2) early transcription is normal but late transcription is delayed and reduced, (3) DNA replication appears normal, and (4) the Mu C gene, whose product positively regulates Mu late genes, is one of the gem target sites. Transcription of a C promoter-lacZ fusion, carried by the pPH91 plasmid, is stimulated both after infection with Mu gem+ or Mu gem3 and in strains lysogenic for the same phages in the presence of viral immunity. These data suggest that the primary role of the gem product is modulation of gene expression. This control could be carried out by direct interaction with transcription factors or by changing DNA supercoiling.