Use of a gene encoding a suppressor tRNA as a reporter of transcription: analyzing the action of the Nun protein of bacteriophage HK022.

The Nun protein of phage HK022 blocks the expression of genes that lie downstream of the nut sites of phage lambda. Nun is believed to act by promoting premature termination of transcription at or near these sites. To test this hypothesis and to facilitate mapping the sites of termination, we inserted a gene encoding a suppressor tRNA immediately downstream of the lambda nutL site and determined the effect of Nun on tRNA level. We found that Nun severely reduced the accumulation of mature, biologically active tRNA and promoted the accumulation of short, promoter-proximal transcripts whose 3' ends were dispersed over a 100-nucleotide region downstream of nutL. These results are consistent with the hypothesis that Nun terminates transcription within the region immediately downstream of nutL and are inconsistent with the hypothesis that the only action of Nun is to prevent translation of genes located downstream of the nut site. The stability, small size, and easily assayable biological function of suppressor tRNA recommend it as a reporter of transcription in other systems.