Mutational analysis of the coordinate expression of the yeast tRNAArg-tRNAAsp gene tandem.

tRNA genes occur in the yeast genome as highly dispersed and independent transcriptional units. The 5'-tRNAArg-tRNAAsp-3' gene tandem, separated by a 10-base-pair spacer sequence, thus represents a rare case of tight clustering. Previous in vitro studies did not reveal any primary transcript from the tRNAAsp gene, but rather a dimeric precursor containing both gene sequences plus spacer, which undergoes a series of maturation steps. This seems anomalous since the tRNAAsp gene contains the sequences necessary for its own transcription. We found that site-directed mutation of the highly conserved C at position 56 to a G in the tRNAArg gene suppresses all transcription and does not activate the tRNAAsp gene. Precise deletion of the entire tRNAArg gene gives a similar result. Rescue of tRNAAsp gene transcription is effected either by the precise deletion of both the tRNAArg gene and spacer or by the precise deletion of this gene with concomitant introduction of an artificial RNA polymerase III start site in the spacer. This artificial start site is ineffective if the tRNAArg gene is present upstream.