Relationship between translation-control and reduced polyadenylation of mRNA in elongated roots of pea seedlings

A large portion of newly formed mRNA transported into the cytoplasmwas found as free mRNPs with discrete S-values of 50-55S, 35-38Sand 22-26S in roots of elongated pea seedlings, in contrastto the younger embryo axis, in which most of the newly formedmRNA was associated with the polysomes. Nitta's finding withVicia seedlings, that free mRNPs contain lesser amounts of oligo(dT)-boundpoly(A)-segments (9), was confirmed by analyzing RNAs from thepolysomes and free mRNPs from elongated pea seedling roots thathad been labeled with ³H-adenosine under the selective inhibitionof rRNA synthesis by 5-FU. A clear difference was observed betweenthe young embryo axis and the elongated seedling root in thedistribution of ³H-adenosine-labeled mRNA in the microsomal(P-30) and postmicrosomal (S-30) fractions. The relative ratioof radioactivity associated with the microsomes was lower inseedling roots. The decrease during germination of the percentof the labeled whole cell RNA binding to the Millipore filter,reported in a previous paper, was proved to be the consequenceof reduced polyadenylation of poly(A)-segments after ³H-adenosine-labeledRNA had been treated with RNase A and RNase T₁ and analyzedon polyacrylamide gels. The evidence showed a close correlationbetween the accumulation of free mRNPs and reduced growth ofpoly (A)-segments in the mRNA of seedling roots. The relationshipbetween the ability of mRNPs to associate with the endoplasmicreticulum and the degree of polyadenylation of mRNA is discussed. (Received January 16, 1979; )