The nucleotide base sequence complexity of total poly(A) RNA from rat tissues and cultured cells of neural origin was measured by RNA-driven hybridization reactions with nonrepetitive [3H]DNA. At saturation, liver and whole brain poly(A) RNAs were complementary to an average of 6.8 and 12.3%, respectively, of the nonrepetitive DNA. Assuming asymmetric transcription, the complexity of liver and brain poly(A) RNAs was equivalent to 3.6 and 6.5 X 108 nucleotides, respectively. The complexity of whole brain poly(A) RNA was similar to that from major functionally distinct brain regions: cerebellum, hypothalamus, hippocampus, and cerebral cortex.Considerations of experimental variance suggest that total poly(A) RNA sequence overlap between major brain regions is substantial and may be >80%. Cultured cells of neural origin had poly(A) RNA complexities which were intermediate between liver and brain in the order: liver < primary glial cultures from 3-day-old brain (predominantly astrocytes) < neuroblastoma (B104) < glioma (C6) < brain. Flybridization of combinations of RNA from the sources indicates extensive similarities in poly(A) RNA sequences between neuroblastoma, glioma, and glial cells and suggests that most liver and J. he nucleotide base sequence complexity of brain RNA has been estimated by hybridization experiments in which nonrepetitive DNA is reacted with excess total RNA (Brown and Church, 1971;Grouse et al., 1972) or nuclear RNA (Hahn and Laird, 1971; Cutler, 1975). Results of these studies suggest that the transcriptional diversity in brain was significantly greater than in other tissues or organs. Recent measurement of the sequence complexity of brain nuclear RNA indicates that 30-40% of the nonrepetitive genome is transcribed in this organ, assuming asymmetric transcription Chikaraishi et al., 1978;Grouse et al., 1978). In contrast, estimates of the nonrepetitive sequence representation in nuclear RNA from other tissues are substantially lower, e.g., 22% in liver and 9-10% for kidney, spleen, and thymus (Chikaraishi et al., 1978).Correspondingly, brain polv(A) mRNA hybridizes to two to three times more nonrepetitive DNA than does mRNA of other tissues or organs. Assuming asymmetric transcription, rodent brain poly(A) mRNA has a sequence complexity of f From the Andrus Gerontology Center and the
