SEQUENCE COMPLEXITY OF NUCLEAR AND CYTOPLASMIC RIBONUCLEIC-ACIDS FROM CLONAL NEUROTUMOR CELL-LINES AND BRAIN SECTIONS OF THE RAT

The seqeunce complexity of both nuclear and cytoplasmic RNA from rat brain is higher than the sequence complexity of these RNA from other tissues. A higher percentage of the nuclear RNA sequences appear as cytoplasmic RNA in brain than in other organs. A study was conducted to determine if the high sequence complexity of brain RNA is found in every cell of the brain or if this high complexity is due to the summation of lower, incompletely overlapping, complexities in the various cell types of the brain. The sequence complexity of RNA fractions from both neurotumor cell lines and brain sections was measured by saturation hybridization. Nuclear RNA from the CNS cell lines B103 and B27 hybridized to 14.5 and 13.5% of the unique-sequence DNA, respectively, while nuclear RNA from the peripheral neurotumor cell line RT4-D1 hybridized to 11.1% of the unique-sequence DNA. If asymmetric transcription is assumed, these hybridization values correspond to complexities of 29.0, 27.0 and 22.2% of the unique-sequence genome or 5.3 .times. 108, 4.9 .times. 108 and 4.0 .times. 108 nucleotides for the 3 cell lines. The sequences transcribed into nuclear RNA in these cell lines are included in the set of sequences transcribed in whole brain. Nuclear RNA from the 5 brain sections examined hybridized to from 14.7-16.3% of the unique-sequence DNA, within experimental error of the 15.6% found for whole brain. Total cytoplasmic RNA from B103 and B27 cell lines hybridized to 3.1 and 4.3% of the unique-sequence DNA, respectively, while released polysomal RNA hybridized to 3.2 and 3.9% for the 2 cell lines. These cytoplasmic complexities are equivalent in each case to .apprx. 25% of the sequences appearing in the nucleus of the cell line. Total cytoplasmic and released polysomal RNA from whole brain represent 50-65% of the sequences appearing in the nucleus. Some brain sections show complexities for polyadenylated cytoplasmic RNA lower than the value for whole brain. This RNA fraction hybridized to 2.8% of the unique-sequence DNA for cerebellum as compared to 5.1% for whole brain. Apparently, most of the sequences transcribed into nuclear RNA of the brain are found in every brain cell, but only 1/4-1/3 of these sequences are eventually translated in any given cell. There must be a great deal of posttranscriptional selection in the brain.