| dc.description | The cytoplasmic ribosome of Crithidia fasciculata, a trypanosomatid protoctist, possesses an unusual complement of rRNAs. Besides an unusually long small subunit (SSU) rRNA of 2206 nucleotides and a heat-labile "28S" rRNA comprised of two high molecular weight (c and d) species, there are four novel small rRNAs (Sp. e, f, g, and j), in addition to the conventional 5S (Sp. h) and 5.8S (Sp. i) rRNAs, which are also present. All together, nine separate rRNAs are present in the C. fasciculata cytoribosome. All except the 5S rRNA are encoded in an 11,373-bp rDNA cistron, which can be isolated as five PstI-rDNA fragments. The genes are arranged in a basically eukaryotic fashion (i.e., 18S-5.8S-28S): starting with the SSU rRNA gene and going 3$\sp\prime$, there is an Internal Transcribed Spacer (ITS1), followed by large subunit (LSU) rRNA coding and spacer regions in the order Sp. i - ITS2 - Sp. c - ITS3 - Sp. e - ITS4 - Sp. d - ITS5 - Sp. f - ITS6 - Sp. j - ITS7 - Sp. g, and finally the Intergenic Spacer (IGS). The rDNA is either an extra-chromosomal circle or genomically integrated and tandemly repeated. By primary and secondary structure comparisons, the novel small rRNAs can be shown to be part of the conventional LSU rRNA. The rRNAs are most likely transcribed from a single promoter in the IGS and produced by post-transcriptional processing. The IGS has a complex physical substructure, the most conspicuous feature of which is an array of 27, 19-nucleotide long, tandem repeats. There are 10 types of these repeats and when compared to one another they show differing degrees of similarity. Their role, if any, in transcription initiation or termination, rDNA replication, or rDNA evolution remains to be elucidated. Flanking this array are three related palindromic sequences, Blocs A, B, and C. They are 40 nucleotides long and could potentionally form hairpin structures in the DNA (or in the RNA if they are transcribed). The rDNA exhibits a length heterogeneity that is stably inherited. There appear to be 6 or 7 size classes of rDNA cistrons differing from one another by a variable amount (200-300 bp). The region encompassing this heterogeneity has been localized to the IGS and is unstable in rec$\sp{+}$ strains of E. coli, giving rise to specific elections. The rDNA seems to be devoid of methylation. | en_US |
