Restriction analysis of tandemly repeated yeast ribosomal RNA genes

Isolation and characterization of ribosomal DNA variants from Sciara coprophila

The ribosomal RNA multigene family in the fungus fly Sciara coprophila contains a total of only 65 to 70 repeat units. We explored the types and frequencies of variant repeats in this small multigene family by characterizing different cloned rDNA variants from Sciara. Although we did not observe any intergenic spacer length variants in Sciara, we found a variant due to the insertion of a putative mobile element (lambda Bc11), and variants containing ribosomal insertion elements. By DNA sequence analysis of rDNA/non-rDNA junctions, there are three distinct types of ribosomal insertion elements found in Sciara rDNA: two correspond to the R1 and R2 insertion elements found in other dipterans (clones lambda Bc5 and pBc1L1, respectively), and one is a novel class of ribosomal insertion elements (R3, exemplified by clone pBc6D6) which so far is unique to Sciara. Together, the several different rDNA variants make up from 12 to 20% of the rDNA in Sciara. These results are discussed in the context of evolution of the ribosomal RNA multigene family.

Unequal crossing over in the ribosomal DNA of Saccharomyces cerevisiae

Unequal sister chromatid exchanges occur at the ribosomal DNA locus of yeast during mitotic growth. The frequency of unequal crossing over, as measured by the deletion or duplication of an inserted genetic marker (LEU2), is sufficient to maintain the sequence homogeneity of the rDNA repeat units.

A study of the organisation of the ribosomal ribonucleic acid gene cluster of Neurospora crassa by means of restriction endonuclease analysis and cloning in bacteriophage lambda

1. Total Neurospora crassa DNA was restricted with endonucleases and fragments carrying rRNA coding sequences were identified by hybridization with Xenopus laevis ribosomal DNA probes. 2. The repeating unit of the rRNA gene cluster was found to be 8.6 kbp, arranged in a head-to-tail fashion. 3. Digestion with Hind III yielded fragments of 3.4 kbp and 5.2 kbp and both were cloned. 4. Digestion with Eco RI yielded fragments of 2.2 kbp, 3.0 kbp and 3.4 kbp; the 3.0 kbp fragment was cloned. 5. Sequences coding for RNA (S-rRNA)1 of the smaller subribosomal particle were found (at least 90%) in the 2.2 kbp EcoRI subfragment of the 5.2 kbp Hind III fragment. 6. The coding sequences for the major RNA species (L-rRNA) of the larger subribosomal particle were located mainly (at least 95%) in the 3.4 kbp Hind III fragment. 7. For comparison, a Hind III digest of total yeast DNA was cloned and recombinants containing a 6.4 kbp rDNA fragment were isolated.