Using [14C]
adenine as a labeled precursor, the biosynthesis and processing of
heterogeneous nuclear RNA (
hnRNA) in
chromatin and nucleoplasm were studied. It was shown that intraperitoneally injected
actinomycin D (50-500 micrograms/100 g of
body weight) did not inhibit the biosynthesis of
hnRNA and rRNA in rat thymocytes. Besides biosynthesis, the polyadenylation of the bulk of primary transcripts was also localized in the
chromatin. rRNA precursors constituted only a small part of the newly synthesized
hnRNA molecules. During processing, highly polymeric (28S-55S)
poly(A+)-
hnRNA fractions passed from
chromatin to the nucleoplasm, as a result of which a large number of high molecular weight polyadenylated
hnRNA molecular were pooled in the nucleoplasm. Thymocyte
hnRNA was tightly bound to nuclear structures and its isolation tributed stringent conditions. The bulk of
poly(A+)-
hnRNA molecules were degraded without formation of stable intermediates. The pattern of
poly(A-)-
hnRNA maturation in
chromatin and nucleoplasm was identical, i.e., in both compartments the decrease in the size and accumulation of processed 16S-20S
RNA molecules was observed. About 6% of newly synthesized
poly(A+)-
hnRNA was converted into polysomal
mRNA. Cytoplasmic
poly(A+)-RNA consists of two fractions, i.e., metabolically stable high molecular weight (greater than or equal to 40S)
RNA molecules and a less stable 10S-20S
RNA.