Depletion of the essential poly(A) binding protein (PAB) in S. cerevisiae by promoter inactivation or by the utilization of a temperature-sensitive mutation (pab1-F364L) results in the inhibition of translation initiation and poly(A) tail shortening. Reversion analysis of pab1-F364L yielded seven independent, extragenic cold-sensitive mutations (spb1-spb7) that also suppress a PAB1 deletion. These mutations allow translation initiation without significantly changing poly(A) tail lengths in the absence of PAB, and they affect the amount of 60S ribosomal subunit. Consistent with this, SPB2 encodes the ribosomal protein L46. These data suggest that the 60S subunit mediates the PAB requirement of translation initiation, thereby ensuring that only intact poly(A)+ mRNA will be translated efficiently in vivo.
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