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E. coli lysogenic for the temperate, λ-related phage HK022 do not support λ growth. The exclusion of λ is caused by the HK022 nungene product, which blocks the expression of genes located downstream of and in the same transcription unit as the λ nutL and nutR sequences. Transcripts terminating prematurely at or near nutR have been detected after inactivation of λ repressor in λ, HK022 dilysogens. Nun therefore appears to be a transcription termination factor with a remarkable specificity; it converts the λ nutsequences, which normally interact with λ N protein to suppress transcription termination, into terminators. These and other similarities between Nun-promoted termination and N-promoted antitermination argue strongly that the mechanisms of the two reactions have steps in common.