Nus A is involved in transcriptional termination on lambda tI

The transcriptional terminator tI generates the 3'end of the integrase (int) gene transcript that is read from the lambda PI promoter in lambda phage. We have studied the factors that affect transcription termination in vitro and in vivo at the lambda tI terminator. In vitro transcriptional studies showed that tI is about 80% efficient in the presence of purified NusA protein, whereas it is only about 50% efficient in its absence. In vivo studies, where the readthrough transcript of lambda tI was measured by quantitative dot blot analysis, gave about 80% efficiency in wild-type strains, but only 60% in the nusA1 mutant strain at non-permissive temperatures. These results support the idea that termination at lambda tI in vivo involves interaction with the NusA factor.