The ribosomal RNA of the trypanosomatid protozoan Crithidia fasciculata: physical characteristics and methylated sequences

Abstract

When extracted and analyzed under conditions which maintain noncovalently associated RNA–RNA complexes, the bulk cellular RNA of Crithidia fasciculata contains species of apparent molecular weights 1.3, 0.825, 0.08, 0.065, and 0.045 × 106 in addition to 5 S rRNA and tRNA. Heat denaturation results in the disappearance of the 1.3 × 106 dalton RNA and the appearance of three new species having molecular weights of 0.67, 0.575, and 0.059 × 106. In addition, the apparent molecular weight of the 0.825 × 106 dalton component is reproducibly lowered to 0.81 × 106 after heat treatment. With the exception of tRNA, all of the RNA species are present in close to equimolar amounts in either undenatured or heat-denatured C. fasciculata bulk cellular RNA. On the basis of previous observations on the ribosomal RNA of the closely related organism, Crithidia oncopelti (Spencer, R. & Cross, G. A. M. (1976) J. Gen. Microbiol. 93, 82–88), the 1.3 and 0.825 × 106 dalton RNA's are considered to be components of the large and small subunits, respectively, of C. fasciculata ribosomes, but the subunit localization of the other RNA's described here has not yet been determined.O2′-Methylnucleosides account for about 1.4 mol% of the total nucleoside constituents of unfractionated C. fasciculata rRNA. Quantitative analysis suggests that the rRNA molecules in a C. fasciculata ribosome contain a total of 95–100 O2′-methyl groups, distributed in 80–85 Nm-Np sequences (including four 'hypermodified' Nm-Np, each containing a modification of a base or base–sugar linkage in addition to sugar methylation), six different Nm-Nm-Np sequences, and one Nm-Nm-Nm-Np sequence. While the specific pattern of O2′-methylation in the rRNA of C. fasciculata is distinct, both qualitatively and quantitatively, from the pattern observed in other organisms, Crithidia rRNA does contain certain 'universal' O2′-methylated sequences which appear to have been extensively conserved in evolution.The base-methylated nucleoside, N6, N6-dimethyladenosine(m26A), has been isolated from both C. fasciculata and wheat embryo rRNA in the form of the alkali-resistant dinucleotide, m26A-m26Ap. This dinucleotide and its enzymatic degradation products have been characterized by examination of their ultraviolet absorption spectra and electrophoretic and chromatographic properties.