DEGRADATION OF CYC1 MESSENGER-RNA IN THE YEAST SACCHAROMYCES-CEREVISIAE DOES NOT REQUIRE TRANSLATION

Several studies have indicated that degradation of certain mRNAs is ti ghtly coupled to their translation, whereas, in contrast, other observ ations suggested that translation can be inhibited without changing th e stability of the mRNA. We have addressed this question with the use of altered CYC1 alleles, which encode iso-l-cytochrome c in the yeast Saccharomyces cerevisiae, The cyc1-1249 mRNA, which lacks all in-frame and out-of-frame AUG triplets, was as stable as the normal mRNA, This finding established that translation is not required for the degradat ion of CYC1 mRNAs, Furthermore, poly(G)ls tracks were introduced withi n the CYC1 mRNA translated regions to block exonuclease degradation, T he recovery of 3' fragments revealed that the translatable and the AUG -deficient mRNAs are both degraded 5' --> 3', Also, the increased stab ility of CYC1 mRNAs in xrn1-Delta strains lacking Xrn1p, the major 5' --> 3' exonuclease, established that the normal and AUG-deficient mRNA s are degraded by the same pathway. In addition, deadenylylation, whic h activates the action of Xrn1p, occurred at equivalent rates in both normal and AUG-deficient mRNAs. We conclude that translation is not re quired for the normal degradation of CYC1 mRNAs, and that translatable and untranslated mRNAs are degraded by the same pathway.