Relationship between nuclear and polysomal RNA populations of Achlya: a simple eucaryotic system

The relationship between hnRNA and mRNA in the water mold Achlya has been investigated in several ways. Analysis of the nuclear and polysomal poly(A) RNA by sucrose density gradient centrifugation under denaturing and nondenaturing conditions showed that the populations have indistinguishable size distributions. The number average sizes were calculated to be 1150 nucleotides for nuclear and 1140 nucleotides for polysomal poly(A) RNA. Selective inhibition of rRNA synthesis was used to investigate the size distribution of hnRNA without regard to poly(A) content. Very little hnRNA was observed which sedimented more rapidly than polysomal poly(A) RNA. Hybridization experiments in which an excess of nuclear DNA was reacted with 3H-poly(A) hnRNA or 3H-poly(A) mRNA showed that both populations contain repetitive transcripts (9-10%) as well as single-copy transcripts (44%). Analysis of hybrids on hydroxyapatite in the presence of 8 M urea demonstrated that the poly(A) RNA complementary to repetitive DNA sequence components represented a population of molecules distinct from the population complementary to single-copy DNA. The complexity of whole cell, nuclear and polysomal RNA was determined by saturation hypbridization to single-copy 3H-DNA. All three populations were complementary to essentially the same fraction of the DNA. Terminal hybridization values were 3.84, 3.76 and 3.76% for whole cell, nuclear and polysomal RNA, respectively, representing a complexity of 2.1 X 10(6) nucleotides. These data suggest that the composition of the hnRNA and mRNA populations are essentially identical. No evidence for selective turnover of any sequence component or size class within the nucleus was observed.