Sequence changes in coliphage lambda mutants affecting the nutL antitermination site and termination by tL1 and tL2.

The 17-bp sequence designated nutL is required for the N-mediated antitermination of transcription in the major leftward operon of coliphage lambda. The single-stranded sequence can be folded into a hairpin structure. Ten independently isolated spontaneous lambda nutL- mutants have changes that affect the same nucleotide, located in the loop of the hairpin structure, changing the guanine to adenine, thymine or cytosine. Another mutant (lambda nutL3), selected by a different means, has a deletion of one GC base pair and thus eliminates one C in the stem of the hairpin structure, destabilizing it -11.2 to -2.2 kcal/mol. True reversions of the nutL point mutations restore the guanine. The second-site revertant lambda ninL99 was found to have a deletion of 417 bp between the tL1 terminator and the N gene, removing bases +523 to +939 (counted from SL = +1). This deletion include codons for the six carboxy-terminal amino acids of gene N product, but the fusion allows continuation of translation for 53 additional amino acid residues beyond the truncated N gene before reaching a nonsense codon. The fused N product is active.