N-Independent leftward transcription in coliphage lambda: Deletions, insertions and new promoters bypassing termination functions

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Abstract
Summary Lambda mutants capable of N-independent red-gam gene expression were isolated by selecting Fec+ plaque-forming derivatives of λN + nutL- (Fec-) strains. In addition to true nutL+ reversions, three classes of second-site mutations were identified: (1) ninL deletions that remove a region containing either t L1 or both t L1 and t L2 termination signals, or only a small region (defining the rut site) just upstream from t L1, (2) new constitutive promoters that map just upstream from the t L2 termination site and which are created either by point mutations (hip) or by short insertion sequences (isp), (3) small internal deletions in gene cro. The positions and individual effects of these mutations, some of which only partially abolish termination function, provide evidence for a complex multipartite structure of the termination signals.