Relationship between nuclear giant-size dRNA and microsomal dRNA of rat brain.

(1) Among the DNA-like RNA (dRNA) species of rat brain, 35 per cent remain nuclear, whereas 65 per cent are found in the microsomes and may represent functional messenger RNA. It was estimated that 30,000-300,000 cistrons would thus code for messenger RNA in rat brain.(2) No nuclear-specific dRNA with sedimentation characteristics similar to microsomal dRNA are detected.(3) The nuclear large dRNA contain all the dRNA species of the brain.(4) The presence of microsomal dRNA precursors and of nuclear restricted dRNA in large dRNA suggests the existence of a limited cleavage process and of a selection among the fragments for their transport from the nucleus to the cytoplasm. Such a selective mechanism could represent an important regulation step in the transfer of genetic information.