The structure and biosynthesis of fungal cytoplasmic ribosomes

Segregation of heterogeneous rDNA segments during demagnification of a Neurospora crassa strain possessing a double nucleolar organizer

We have produced a duplication strain of Neurospora crassa, DpAR33-PR-6, which contains two cytologically visible nucleoli (a DNO or double nucleolar organizer strain). When freshly generated, this strain has approximately twice the number of rRNA cistrons found in the parental (single nucleolar organizer) strains. After several serial propagations, there is a marked reduction in rRNA cistron number, approximating that of the SNO parental strains. This reduction in rRNA cistrons ("demagnification") was not achieved by breakdown of the VL→IVL translocation used to generate the duplication, as rDNA from the two parents can be distinguished by the size of the non-transcribed spacer region in the rDNA repeat unit of each strain. rDNA characteristic of both parents is present even after demagnification, in approximately equal amounts, suggesting the rRNA cistrons are lost randomly and non-preferentially from each homologous chromatid. In addition, the steady-state growth rate appears to be affected by rRNA cistron number, decreasing in freshly generated DNO strains relative to the parental strains, and returning to parental levels after demagnification.

RRP1, a Saccharomyces cerevisiae gene affecting rRNA processing and production of mature ribosomal subunits

The Saccharomyces cerevisiae mutant ts351 had been shown to affect processing of 27S pre-rRNA to mature 25S and 5.8S rRNAs (C. Andrew, A. K. Hopper, and B. D. Hall, Mol. Gen. Genet. 144:29-37, 1976). We showed that this strain contains two mutations leading to temperature-sensitive lethality. The rRNA-processing defect, however, is a result of only one of the two mutations. We designated the lesion responsible for the rRNA-processing defect rrp1 and showed that it is located on the right arm of chromosome IV either allelic to or tightly linked to mak21. This rrp1 lesion also results in hypersensitivity to aminoglycoside antibiotics and a reduced 25S/18S rRNA ratio at semipermissive temperatures. We cloned the RRP1 gene and provide evidence that it encodes a moderately abundant mRNA which is in lower abundance and larger than most mRNAs encoding ribosomal proteins.

Magnification of rRNA gene number in a Neurospora crassa strain with a partial deletion of the nucleolus organizer

Some progeny from crosses between the Neurospora crassa translocation strain T(IL----VL)OY321 and normal sequence N. crassa strains are duplication strains with a partial deletion of the nucleolus organizer. Despite the deletion, these progeny are viable and produce a functional nucleolus. Quantification of rRNA gene number in these deletion progeny demonstrated a significant loss of rRNA genes, down to 60% of the parental wild-type level. Initially, all of these reduced nucleolus organizer (RNO) strains demonstrated a reduction in the rate of mycelial elongation in growth tubes. After several vegetative growth cycles some progeny reverted to the normal growth phenotype, and also showed an increase in the number of rRNA genes to approximately that of the wild type.