3′ Termini of Mammalian and Insect Mitochondrial rRNAs

Abstract

We have presented evidence that the 3′ terminus of the large ribosomal subunit (LSU) RNA of hamster (BHK-21) mitochondria, 17S RNA, is distinctive. Most molecules contain 3′ -terminal oligoadenylate moieties, which we presume to be added posttranscriptionally; and the putative transcribed moieties are “ragged,” ending at any of several residues (Dubin et al. 1981). We have now defined the 3′ termini of 17S RNA more precisely, and we have done comparative studies on the small subunit (SSU) RNA of hamster cells, 13S RNA; on the homologous human rRNAs; and on the homologous RNA of mosquito (Aedes albopictus) cells.

17S RNA TERMINI
Our approach to characterizing terminal sequences was similar to that described earlier (Dubin et al. 1981). Figure 1 shows a representative fingerprint from an RNase-T1 digest of 3′-end-labeled 17S RNA. Several of the larger oligonucleotides were sequenced using partial exonuclease digestion and mobility-shift analysis. The sequences of oligonucleotides 30 and 21 were found to be UUUAUUAGp and UUUAUUACp, respectively. Spot 21 is now seen to be the most plentiful member of a major oligoadenylated family, UUUAUUA_nCp (spots 20–25); the adenylated congeners of spot 30 appear as A_nCp in RNase-T1 digests. We presume that the minor family between the “20” and the “11” families is UUUAUA_nCp. This would indicate that the 11 family is U₃A_nCp rather than U₂A_nCp (cf. Dubin et al. 1981), and mobility-shift analysis confirmed this. Together with an AGGA_nCp family identified in RNase-A digests, these oligonucleotides can be overlapped to form a putative 3′-terminal-transcribed moiety, GUUUAUUAGGUUU...