SEQUENCE DIVERSITY STUDIES OF RAT BRAIN RNA: EFFECTS OF ENVIRONMENTAL COMPLEXITY ON RAT BRAIN RNA DIVERSITY

— The sequence complexities of rat brain RNAs were measured by RNA‐driven hybridization reactions with nonrepetitive rat DNA. The total sequence complexity of rat brain HnRNA was estimated to be 6.61 x 10 8 nucleotides while rat brain poly(A)‐mRNA sequence complexity was 1.32 x 10 8 nucleotides. Up to 33.7% of the total transcribable nonrepetitive DNA was expressed in the nuclear RNA. The nuclear RNAs reacted with complex kinetics over at least 4.5 decades of equivalent Rot (product of RNA concentration and time), with an apparent division into three major RNA abundance classes. The abundances of average nuclear RNA species in these classes ranged from 2.9 x 10 9 copies per brain (18 copies per cell) to 2.4 x 10 5 copies per brain (1.5 x 10 −3 copies per cell). Poly(A)‐mRNA diversity was sufficient to code for 8.8 x 10 4 polypeptides of 50,000 daltons. There were also three distinguishable abundance classes of poly(A)‐mRNA with frequencies which ranged from 8.9 x 10 8 copies per brain (5.5 copies per cell) to 3.2 x 10 5 copies per brain (2 x 10 −3 copies per cell). Evidence for compartmentalization of expressed RNA sequences supports the concept that the extensive morphological and physiological specialization evident in brain parallels extensive transcriptional specialization at the cellular level. Brain and liver RNA diversities were measured under double‐blind experimental conditions in three experiments with rats raised in experientially enriched (EC) or impoverished (IC) environments. Liver RNA diversity of EC animals was not different from that of IC animals. Brain total RNA of EC animals, at equivalent R 0 ts of 184,000‐212,000, hybridized to 10.6% of rat unique DNA (mean of 11 separate groups of rats). The average hybridization of brain RNA from 11 groups of IC animals in the same range of equivalent Rot was 8.2% of the unique DNA. The difference was statistically significant at P < 0.02. Of 10 groups of 3 littermate pairs (paired across EC and IC groups) brain RNA diversity was greater in EC animals in 8 cases. A least squares fit of the kinetics of hybridization to a pseudo first order reaction showed that, at saturation, the RNA from brains of EC animals was complementary to 16.4% of the unique DNA while that from IC animals was complementary to 9.1%. This difference was found in the least abundant class of rat brain RNA. These changes in sequence diversity reflected either an increase in the number of diverse RNA species present or an increase in the number of copies of certain RNA species in the rats raised in an enriched environment. A change in brain RNA populations of this magnitude may reflect a significant difference in brain function between EC and IC animals.