CLONING OF DNA CORRESPONDING TO RARE TRANSCRIPTS OF RAT-BRAIN - EVIDENCE OF TRANSCRIPTIONAL AND POST-TRANSCRIPTIONAL CONTROL AND OF THE EXISTENCE OF NONPOLYADENYLATED TRANSCRIPTS

To examine the expression of genes encoding rare transcripts in the rat brain, one characterized genomic DNA clones corresponding to this class. In brain cells, as in all cell types, rare transcripts constitute the majority of different sequences transcribed. When compared with other tissues or cultured cells, brain tissue may be expected to have an even larger set of rare transcripts, some of which could be restricted to subpopulations of neural cells. Seven clones whose transcripts are nonabundant, averaging < 3 copies/cell, were identified. Clone rg13 (rat genomic 13) RNA was detected only in the brain, whereas RNA of a 2nd clone, rg40, was also detected in the brain and in a melanoma. Transcripts of rg13 were found in cerebellum, cerebral cortex and regions underlying the cortex, whereas rg40 transcripts were not detected in the cerebellum. Transcripts of both rg13 and rg40 were found in pelleted polysomal RNA. RNA of another clone, rg34, was found in the brain, liver and kidney but was found in pelleted polysomal RNA only in the brain, suggesting that its expression may be post-transcriptionally controlled. The remaining 4 clones represent rare transcripts that are common to the brain, liver and kidney; rg18 RNA is restricted to the nucleus, whereas rg3, rg26 and rg36 transcripts are found in the cytoplasm of all 3 tissues. Transcripts of the brain-specific clone, rg13, and the commonly expressed cloned, rg3, are nonpolyadenylated, presumably belonging to the high-complexity, nonpolyadenylated class of transcripts in the mammalian brain.