In S. cerevisiae, thermosensitive mutations in the unlinked genes RNA14 and RNA15 cause very similar phenotypes once the cells are shifted to the non permissive growth temperature. The length of the mRNA poly(A) tails is shortened and the mRNA stability is strongly decreased whilst the polymerase II transcription rate is only slightly modified in an allelic-dependent manner (Bloch et al., 1978, Minvielle-Sebastia et al., 1991). Moreover there is in both mutants a significant increase of monosomes concomitant with a strong reduction in the polysome population (A. Petitjean, unpublished results). This could either reflect the mRNA instability or a more specific impairment of these mutants in translation initiation (Petersen and Mc Laughlin, 1974).
