METABOLISM OF OLIGO(A) TRACTS AND ASSOCIATION OF OLIGO(A)(+)RNA WITH POLYSOMES IN MOLLUSCAN EMBRYOS

Six-day preveliger embryos of the marine gastropod Nassarius (Ilyanassa) obsoletus were labeled with [3H]adenosine, and RNA was extracted and then digested with DNase I, RNase A, and RNase T1 under conditions which leave adenosine homopolymers intact. Nuclease-resistant material was characterized as poly(A) and oligo(A) by analysis of size distribution, affinity chromatography on oligo(dT)-cellulose, and sensitivity to RNase T2. Oligo(A) tracts, less than 30 nucleotides long, were identified which incorporate adenosine but not uridine, and are resistant to RNases A and T1 but sensitive to T2; those tracts less than 20 nucleotides long were not retained on oligo(dT)-cellulose and could then be obtained free of poly(A). After a 6-h labeling period, about 80% of nuclease-resistant label from postnuclear supernatant fractions was in oligo(A). Alkaline hydrolysates of Ilyanassa poly(A) had the adenosine content expected of 3''-poly(A), whereas oligo(A) hydrolysates contained little adenosine, indicating their location elsewhere in RNA molecules. Of the 2 principle size classes of oligo(A), 8 and 18 nucleotides, the 1st was distributed among subcellular fractions, including polyribosomes, in a fashion similar to poly(A). Oligo(A)18, in contrast, was largely localized in molecules or molecular complexes smaller than ribosomal subunits, although it, too, appeared in association with polysomes. Whereas the labeled poly(A) of ribosome pellet fractions underwent turnover, the labeled oligo(A) was stable for at least 10 h. Oligo(A)8 and oligo(A)18 are thus metabolically distinct from each other and from poly(A), and are a prominent feature of cytoplasmic RNA in this system.