EFFICIENT TRANSLATION OF POLY(A)-DEFICIENT MESSENGER-RNAS IN SACCHAROMYCES-CEREVISIAE

The polyadenylate tail of eukaryotic mRNAs is thought to influence var ious metabolic phenomena including mRNA stability, translation initiat ion, and nucleo-cytoplasmic transport. We have analyzed the fate of mR NAs following inactivation of poly(A) polymerase in Saccharomyces cere visiae containing a temperature-sensitive, lethal mutation (pap1-1) in the gene for poly(A) polymerase (PAP1). Inactivation of poly(A) polym erase (Pap1) by shifting cells to the nonpermissive temperature result ed in the loss of at least 80% of measurable poly(A) within 60 min. No rthern blot analysis revealed the disappearance of some mRNAs (CYH2 an d HIS4) consistent with a role for poly(A) tails in mRNA stability. Ho wever, other mRNAs (TCM1, PAB1, ACT1, and HTB2) accumulate as poly(A)- deficient (A<similar to 25) transcripts as defined by an inability to bind oligo(dT)-cellulose. Sucrose density gradient analysis of polyrib osomes revealed a twofold reduction in the amount of each size class o f polyribosomes in shifted cells and a commensurate increase in free r ibosomes. However, poly(A)-deficient mRNAs in shifted cells remain ass ociated with the same size polyribosomes as poly(A)(+) mRNAs in unshif ted cells, indicating normal initiation of translation. RNase mapping of transcripts from pap1-1 cells revealed PAB1 mRNA to be poly(A)(-) w hereas TCM1 exists as equal amounts of poly(A)(-) and poly(A)(+) mRNA 60 min after shift. Interestingly, both of these classes of TCM1 mRNA appear in similar amounts in each polyribosome fraction indicating tha t ribosomes may not distinguish between them. These findings suggest t hat under conditions of excess translational capacity,poly(A)(-) and p oly(A)(+) mRNAs may initiate translation with comparable efficiencies.