The 3'-terminal region of mosquito mitochondrial small ribosomal subunit RNA: sequence and localization of methylated residues.

The 3'-terminal 101 residues of the small ribosomal subunit (SSU) RNA of mosquito cell mitochondria have been determined. This stretch includes the four methylated residues of the molecule: an m4C, an m5C, and two m26A residues. The m26A's occur in a typical m26A "arm," and the methylated Cs in the unique subsequence G x m4C . C . m5C . A, which is homologous in position to a conserved methylated GCCCG subsequence of other SSU RNA classes. There is fairly good overall homology between the mosquito mitochondrial sequence and corresponding regions of other SSU RNA classes, except that a domain of 50-100 residues, previously considered universal, is absent. Comparison with mammalian mitochondrial sequences revealed a marked preponderance of transitional base substitutions, supporting earlier evidence that the 3'-terminal region of SSU RNA is under special structural constraints. The extreme 3' end of the mosquito sequence is heterogeneous, three-fourths of the molecules ending in ... GA and one-fourth ending in ... GAA. Evidence is presented indicating that some, at least, of the 3'-terminal A residues may be added post-transcriptionally, as occurs in mammalian mitochondrial systems. Taken together, the results provide modest support for the monophyletic evolutionary origin of insect and mammalian mitochondria from a primitive procaryotic ancestor.