A branched tetranucleotide consisting of adenosine linked 2' and 5' to guanosine and 3' to cytidine was synthesized from appropriately protected nucleoside phosphoramidites as synthons. The product was characterized enzymatically.

The heterogeneous RNA of the HeLa cell nucleus contains short internal poly(A) sequences which, in contrast to the longer poly(A) sequences at the 3′ ends, are not found in messenger RNA of the cytoplasm. A distinct origin for each of these homologous sequences is evident when the effects of actinomycin D and 3′-deoxyadenosine on their biosynthesis are compared. The shorter poly(A) appears to be transcribed, while the longer one does not. A kinetic analysis of the metabolism of each type of poly(A) sequence within different size classes of HnRNA after treatment of the cells with 3′-deoxyadenosine reveals distinctive distribution and metabolism of these sequences and also provides insight into mechanisms proposed for the generation of messenger RNA from these large RNA molecules in the nucleus.

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A class of heterogeneous nuclear RNA (hnRNA) molecules contain an internal transcribed poly A sequence of close to 25 uninterrupted AMP residues. HnRNA molecules containing this sequence are separable from those containing the large 3' terminal poly A sequence on the basis of their differential affinity for oligo dT cellulose. The fact that the transcribed small poly A and the 3' terminal poly A are not found in the same hnRNA molecules even though both are present in similar size classes and that the small poly A is absent from cytoplasmic messenger RNA (mRNA) has led us to propose a scheme for mRNA processing in which the 3' end of the small poly A in hnRNA becomes a priming size for the post-transcriptional addiction of the large poly A.