Sequence complexity and abundance classes of nuclear and polysomal polyadenylated RNA in Neurospora crassa

The sequence complexity of nuclear and polysomal polyadenylated RNA in Neurospora crassa has been investigated by an analysis of the kinetics of RNA : cDNA hybridizations. There are about 2000 different messenger RNAs organized into three abundance classes of low, medium and high complexity which contain approx. 10, 150 and 1800 sequences, respectively. Taking 1300 nucleotides as the average length of mRNA, the total sequence complexity of polyadenylated polysomal RNA was calculated to represent 2.4 . 10(6) nucleotide pairs, which is 9% of the genome. Hybridization of polysomal polyadenylated RNA with nuclear DNA yielded results in good agreement and revealed that about 12% of the genome was transcribed into mRNA. Analysis of RNA : cDNA hybridizations with nuclear polyadenylated RNA gave results similar to that observed with polysomal RNA, and indicated that nuclear RNAs were also present in discrete abundance classes. Cross-hybridization experiments showed that all mRNA sequences are present in nuclear RNA, and that the sequence complexity detected in polysomal and nuclear polyadenylated RNA is identical or very similar. In total, approx. 15 to 20% of the Neurospora genome is transcribed into various RNA species, including messenger and ribosomal RNA, in cells growing vegetatively on minimal medium.