Neurospora crassa cytoplasmic ribosomes; ribosomal ribonucleic acid synthesis in the wild type

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Abstract
The biosynthesis of rRNA in wild-type N. crassa growing at 25.degree. C was investigated by continuous-labeling and pulse-chase experiments using [5-3H[tritiated hydrogen in the 5 positions] uridine. The following precursor-product relationships are suggested. The 1st RNA molecule to be synthesized in significant quantities is the 2.4 .times. 106-dalton (2.4-Mdal)ribosomal precursor RNA. This RNA is cleaved to produce 2 species of RNA with weights of 0.7 and 1.4 Mdal. The former is the mature 17S rRNA of the 37S ribosomal subunit. The 1.4-Mdal RNA is subsequently cleaved to produce the mature 1.27-Mdal (25S) and 61,000-dalton (5.8S) rRNA of the 60S ribosomal subunit. In the maturation process, approximately 15-20% of the 2.4-Mdal ribosomal precursor rRNA molecule is lost. As in other eukaryotes that have been examined, 5S rRNA is not derived from this precursor molecule.