Expression of a tRNA gene in the context of the lacZ mRNA

Fusions of the gene for tyrosine suppressor tRNA, tyrT(Sup3), and the lacZ gene of Escherichia coli were constructed such that the tRNA gene could be expressed from either its own promoter or that of the lac operon. These chimeras, carried on phage M13 vectors, were tested for the expression of the tRNA in E. coli. The tRNA gene was expressed on the order of 10-fold more weakly from the lac promoter than from its own promoter. To examine whether pausing or premature termination of transcription played a role in determining the relative strength, the fusions were tested in a variety of genetic backgrounds and under different physiological conditions that uncouple transcription and translation. The expression of the tRNA was not enhanced in backgrounds in which polarity was weakened or under the other conditions tested, although a dependence on nusB function was observed when the tRNA was transcribed from the lac promoter. These results indicate that pausing or premature termination of transcription did not play a role in the weak expression of the gene fusions. The results further suggest that the transcription of the tyrT gene does not normally require relief from polarity as imposed by any of the known transcriptional termination systems, in contrast to the antitermination system thought to be involved in the expression of the rRNAs.