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.