COMPLEXITY OF CYTOPLASMIC POLYADENYLATED AND NONPOLYADENYLATED RAT-BRAIN RIBONUCLEIC-ACIDS

Measured by saturation hybridization to radio-iodinated single-copy DNA, rat brain cytoplasmic RNA annealed to 10.0% of the rat unique sequence DNA. By assuming asymmetric transcription, this complexity is equivalent to 20% of the single-copy genome or 3.6 .times. 108 nucleotides, enough information for 240,000 different 50,000-dalton proteins. Half the complexity resides in polyadenylated RNA and half in nonpolyadenylated RNA. Sequential additivity experiments show that poly(A)+ and poly(A)- cytoplasmic RNA contain nonoverlapping sets of sequences. Both RNA populations are composed of at least 2 abundance frequencies; the most abundant class has an average copy number of 5-7 copies/cell, while the rare frequency class is present at an average of 0.014 copy/cell for poly(A)+ RNA and 0.18 copy/cell for poly(A)-RNA. Since saturation hybridization measures primarily high-complexity, low-abundance RNA, comparison of cell-free translation products directed by poly(A)+ and poly(A)- RNA was performed to adjudge sequence similarities between highly abundant RNA. In contrast to the hybridization results, many of the major translation products synthesized in a rabbit reticulocyte translation system directed by poly(A)-RNA are also found among the poly(A)+ translation products, a major common protein being .beta.-actin. Although the most abundant proteins are held in common, the majority of the detectable poly(A)- products are unique to the poly(A)- fraction as analyzed by 2-dimensional gel electrophoresis. To eliminate possible contamination of cytoplasmic RNA by high-complexity nuclear RNA, puromycin-released brain polysomal RNA was prepared. This RNA saturated 8% of probe DNA. Assuming asymmetric transcription, this is equivalent to 2.9 .times. 108 nucleotides or 192,000 different 1500-nucleotide-long mRNAs. By comparison, cytoplasmic liver and kidney RNA have complexities of 8.6 .times. 107 and 5.8 107 nucleotides, respectively, equal to about 57,000 active liver genes and 39,000 active kidney genes.