STUDY OF RNA IN SUBCELLULAR FRACTIONS FROM RAT BRAIN: SIMULTANEOUS INCORPORATION OF [14C]URIDINE AND [3H]METHYL‐ l ‐METHIONINE

By using a combination of subcutaneous and intraventricular injections of [14C]uridine and [3H]methyl‐l‐methionine we have obtained maximum incorporation in about 40 min of both radioactive precursors into nuclear RNA from rat brain. In this nuclear fraction we found at least two different types of RNA that were rapidly labelled. One of them incorporated both [14C]uridine and [3H]methyl groups and seemed to correspond to species of rRNA and their precursors. The other RNA fraction was less methylated or non‐methylated and exhibited sedimentation coefficients distributed along a continuous 8–30 % sucrose density gradient. At least part of the latter type of RNA very probably was mRNA, but much of it must conespond to a different RNA similar to that recently described in HeLa cells by Penman, Vesco and Penman (1968). We also found that labelled 185 and 285 rRNA components began leaving the nucleus for the cytoplasm within 24 to 33 min after the radioactive precursors had been injected, and, in the cytoplasmic fraction, the patterns of incorporation for [14C]uridine and [3H]‐methyl groups were similar for the 18S and 28S rRNA components. We estimate that in this fraction of rat brain the 18S rRNA component was 1·4 times more methylated than the 28S component. We also detected a lower sedimentation coefficient for the non‐ or slightly methylated, species of soluble RNA found in the cytoplasmic fraction.