Suppression of lambda PRM- mutations by cin-1, a mutation creating a new promoter for leftward transcription of the cI gene

Repression of the lambda pcin promoter by integrative host factor

The cin-1 mutation creates a new promoter (pcin) in the tR1 region of bacteriophage lambda. The pcin promoter transcribes the cI repressor gene constitutively. lambda cin-1 does not propagate on Escherichia coli mutants lacking the integrative host factor (IHF). lambda cI- cin-1 grows normally in IHF- mutants, indicating that repressor overproduction from pcin blocks lytic growth. The presence of an IHF binding site which overlaps the pcin promoter led us to the hypothesis that IHF functions as a repressor of pcin transcription. We find that the pcin promoter is fivefold more active in a host lacking IHF than in wild-type cells. In vitro studies show that IHF directly inhibits transcription initiation at pcin. Abortive initiation and gel retardation assays demonstrate that IHF interferes with the binding of RNA polymerase to the pcin promoter. RNA polymerase bound in an open promoter complex is resistant to IHF. We propose that IHF binding to the pcin promoter region blocks the binding of RNA polymerase to the promoter, either by covering specific nucleotides or by distorting DNA structure.

Mutants in the y region of bacteriophage lambda constitutive for repressor synthesis: their isolation and the characterization of the Hyp phenotype

Bacteriophage lambdahyp mutants have been isolated as survivors of Escherichia coli K-12 bacteria lysogenic for lambda Nam7am53cI857. The hyp mutants are characterized by (i) their localization in the y region very close to the imm lambda/imm434 boundary, (ii) polarity on O gene expression, (iii) immediate recovery of lambda immunity at 30 degrees C after prolonged growth of lambda Nam7am53cI857 hyp lysogens at 42 degrees C even in the presence of an active cro gene product, (iv) ability of phage lambda v2v3vs326 but not lambda v1v2v3 to propagate on lambda cI+hyp lysogens, (v) inability to express lambda exonuclease activity after prophage induction, and (vi) inviability at any temperature of phage carrying the hyp mutation. All these properties are referred to collectively as the Hyp phenotype. We show that the Hyp phenotype is due to cII-independent constitutive cI-gene-product synthesis originating in the y region, which results in the synthesis of anti-cro RNA species, and constitutive levels of cro gene product present even in lambda cI+hyp lysogens. A model is presented which is consistent with all the experimental observations.

An IS4 transposition causes a 13-bp duplication of phage lambda DNA and results in the constitutive expression of the cI and cro gene products

We have examined the nature of the additional DNA present in lambda hyp- mutants (Eisen et al., 1982). This DNA is an IS4 element in orientation I, in the y region of bacteriophage lambda at nucleotide position 39,139 (see Moore et al., 1979). Our assignment is based on (i) the similarity in size derived from the PstI, AvaI, and HindII restriction pattern and (ii) the DNA sequence of both the left and right lambda-IS4 DNA junctions in phage lambda hyp15rev4. The IS4 integration event resulted in the duplication of 13 bp of lambda DNA in contrast to the 11- and 12-bp duplications previously observed at the sites of IS4 integrations elsewhere (Klaer et al., 1981).