Expression and inheritance of the yeast extrachromosomal element psi do not depend on RNA polymerase I.

The extrachromosomal element psi affects translation fidelity in the yeast Saccharomyces cerevisiae by increasing the efficiency of tRNA-mediated ochre suppression. The nature of the psi factor is unknown, although there is evidence that 3-microns circles from psi+ strains can be used to transform psi- cells to psi+. The 3-microns circles are extrachromosomal copies of the repeating ribosomal DNA unit, which is organized into two transcription units: the 35s rRNA precursor transcribed by RNA polymerase I, and the 5s rRNA transcribed by RNA polymerase III. We used a strain containing a mutation in RNA polymerase I to test whether psi expression and inheritance depended on RNA polI. Neither expression nor inheritance of psi requires intact RNA polI.