YEAST-CELLS LACKING 5'-]3' EXORIBONUCLEASE-1 CONTAIN MESSENGER-RNA SPECIES THAT ARE POLY(A) DEFICIENT AND PARTIALLY LACK THE 5' CAP STRUCTURE

Analysis of the slowed turnover rates of several specific MRNA species and the higher cellular levels of some of these mRNAs in Saccharomyces cerevisiae lacking 5' --> 3' exoribonuclease 1 (xrn1 cells) has led to the finding that these yeast contain higher amounts of essentially full-length mRNAs that do not bind to oligo(dT)-cellulose. On the other hand, the length of mRNA poly(A) chains found after pulse-labeling of cells lacking the exoribonuclesse, the cellular rate of synthesis of oligo(dT)-bound mRNA, and the initial rate of its deadenylation appeared quite similar to the same measurements in wild-type yeast cells. Examination of the 5' cap structure status of the poly(A)-deficient mRNAs by comparative analysis of the m7G content of poly(A)- and poly(A)+ RNA fractions of wild-type and xrn1 cells suggested that the xrn1 poly(A) - mRNA fraction is low in cap structure content. Further analysis of the 5' termini by measurements of the rate of 5' --> 3' exoribonuclease 1 hydrolysis of specific full-length mRNA species showed that approximately 50% of the xrn1 poly(A)-deficient mRNA species lack the cap structure. Primer extension analysis of the 5' terminus of ribosomal protein 51A (RP51A) mRNA showed that about 30% of the poly(A)-deficient molecules of the xrn1 cells are slightly shorter at the 5' end. The finding of some accumulation of poly(A)-deficient mRNA species partially lacking the cap structure together with the reduction of the rate of mRNA turnover in cells lacking the enzyme suggest a possible role for 5' --> 3' exoribonuclease 1 in the mRNA turnover process.