Regulation of ribosomal RNA synthesis of Drosophila melanogaster
Signature was redacted for privacy. Signature was redacted for privacy. Signature was redacted for privacy. PLEASE NOTE: Some Pages have Indistinct print. Filmed as received. UNIVERSITY MICROFILMS ii ABBREVIATIONS Constitution of Ribosomes Ribosomes are particles found both in the cytoplasm and the nucleus, sometimes attached to membranes,and are involved in protein synthesis. They are made up of RNA and many different proteins. The dimensions of the ribosomes of eukaryotes are 340 x 240 x 240
... and they have a molecu lar weight of 4.1-4.7 x 10^ daltons (SPIRIN and GAVRILOVA, 1969). This unit, which has a sedimentation coefficient of 80s, can be dissociated into a 60s and a 40s subunit. The large subunit (60s) contains a 28s rRNA and, depending on the species, (Review by ATTARDI and AMALDI, 1970) has a molecular weight of 1.3-1.6 x 10^ daltons. The 60s subunit also contains 4 a 5s RNA (approximate molecular weight 4 x 10 daltons) and a 7s RNA. The 7s RNA has been found to be hydrogen bonded to the 28s rRNA and derived from the same precursor as the 28s (ATTARDI and AMALDI, 1970). In the smaller 40s ribosomal subunit, an 18s rRNA is found to have a molecular weight of .69-.89 x 10^ daltons, also varying with the species (ATTARDI and AMALDI, 1970). Some 50 different proteins are associated with the RNA to form the complete ribosome. Although little is known about these basic proteins, they have been partially characterized for Drosophila by acrylamide gel electrophoresis (LAMBERTSON, RASMUSON, and BLOOM, 1970). Synthesis of High Molecular Weight Ribosomal RNA The synthesis of ribosomal RNA in higher organisms involves mainly two steps: First, one of transcription, and second, one of postranscriptional modification. Organization of the Genes for the RTUs There are several copies of RTU genes per haploid genome (RITOSSA and SPIEGELMAN, 1965). It has been calculated from hybridization studies that Drosophila normally has between 130-180 copies of RTU per haploid genome. These are all clustered in the nucleolar organizer region. For convenience, we would like to refer to each cluster of repeated sequences, which is called a redundant region, as a REDON. For example, a diploid * normal Drosophila has two RTU redons with 130 copies each. It is thought that there are fewer promoters than basic sequences, maybe as few as one promoter per redon (PERRY and KELLEY, 1970; ATWOOD, 1969). Variations in redon size are known to occur. The sex-linked bobbed mutants in Dro sophila are known to have RTU redons of reduced size (RITOSSA, ATWOOD, and SPIEGELMAN, 1966). These bobbed mutants have short bristles, etched abdomen, and an increased developmental time (LINDSLEY and GRELL, 1968). Different grades of are found, from very slight (almost impos sible to separate from wild type) up to recessive lethais, the intensity proportional to the extent of the rDNA deletion. The fact that the RTU redon is so large, i..£., has 130 copies, allows partial deletions to survive. It is believed that the RTU redon, the nucleolar organizer region, and the bobbed locus are one and the same thing (RITOSSA, ATWOOD, and SPIEGELMAN, 1966). RITOSSA et al. (1966) proposes that, if less than half the wild type RTUs genes are present, in either one or two redons, a bobbed phenotype ocçurs. MOHAN and RITOSSA (1970) showed Variations depend on the estimate of DNA per haploid genome and on wild type used.