Antitermination and termination functions of the cloned nutL, N, and tL1 modules of coliphage lambda.

A plasmid containing a pp-galK operon was constructed to assay galK expression as a measure of transcriptional regulation by the cloned nutL, N and tL1 modules in a Rho+, Nus+ Escherichia coli host. Insertion of tL1 and rut, both carried on a lambda fragment located between the bamHI site and gene N, between the promoter and galK reduced expression of galK by 80--90%. Gene N alone, when controlled by pp, stimulated galK expression by about two-fold. Cloning of both gene N and nutL in the proper orientation resulted in about a 60% decrease in the tL1 termination. Additional tandem nutL sites increased efficiency of antitermination. A shortened nutL segment, containing only 25 bp of the original genomic nutL sequence, was found to have nearly equal ability to bring about antitermination. If the orientation of the nutL fragment is reversed, antitermination is abolished and the insert now displays a termination function. Termination efficiency is 60 to 73% for one to four tandem nutL modules in reverse orientation. Similarly, the inverted N module acts as a terminator, with 67% efficiency for one and 90% for two tandem inserts. Termination by inverted nutL or N fragments is probably unrelated to their normal functions, but indicates a fortuitous presence of a terminator sequence in the inverted orientation.