COMPLEXITY AND CHARACTERIZATION OF POLYADENYLATED RNA IN MOUSE-BRAIN

The complexity of nuclear RNA, poly(A)hnRNA [heterogeneous poly(A)RNA], poly(A)mRNA, and total poly(A)RNA from mouse brain was measured by saturation hybridization with nonrepeated DNA. These DNA populations were complementary, respectively, to 21, 13.5, 3.8 and 13.3% of the DNA. From the RNA nucleotide concentration required to achieve half-saturation, it was estimated that about 2.5-3% of the mass of total nuclear RNA constituted most of the complexity. Similarly, complexity driver molecules constituted 6-7% of the mass of the poly(A)hnRNA. Poly(A)mRNA diversity (75-80%) is contained in an estimated 4-5% of the mass of this mRNA. Poly(A)hnRNA constituted about 20% of the mass of nuclear RNA and was comprised of molecules which sedimented in dimethyl sulfoxide-sucrose gradients largely between 16S and 60S. The number average size of poly(A)hnRNA determined by sedimentation, EM, or poly(A) content was 4200-4800 nucleotides. Poly(A)mRNA constituted about 2% of the total polysomal RNA, and the number average size was 1100-1400 nucleotides. The complexity of whole cell poly(A)RNA, which contains both poly(A)hnRNA and poly(A)mRNA populations, was the same as poly(A)hnRNA. This implies that cytoplasmic polyadenylation does not occur to any apparent qualitative extent and that poly(A)mRNA is a subset of the poly(A)hnRNA population. The complexity of poly(A)hnRNA and poly(A)mRNA in kilobases was 5 .times. 105 and 1.4 .times. 105, respectively. DNA which hybridized with poly(A)mRNA renatures in the presence of excess total DNA at the same rate as nonrepetitive tracer DNA. Hence saturation values are due to hybridization with nonrepeated DNA and are therefore a direct measure of the sequence complexity of poly(A)mRNA. The nonrepeated sequence complexity of the poly(A)mRNA population is equal to about 1/4 that observed for poly(A)hnRNA.