Bacteriophage P22 helps bacteriophage MB78 to overcome the transcription inhibition in rifampicin resistant mutant of Salmonella typhimurium.

Bacteriophage MB78 cannot grow on rifampicin resistant mutant of host Salmonella typhimurium (rif39) which contains an altered beta subunit of RNA polymerase. Bacteriophage P22, however, grows normally in rif39 both in the presence or absence of rifampicin. Perhaps MB78 promoter is not recognized by altered RNA polymerase. As the phage P22 helps MB78 to grow to some extent on rif39, hybrids between P22 and MB78 have been isolated. Hybrid phage which can grow on rif39 contains mostly genes from MB78 although a small portion (15-20 per cent) of the genome belongs to P22 genome which helps MB78 to overcome the transcription inhibition in the host mutant with altered RNA polymerase.