A CONSERVED HEPTAMER MOTIF FOR RIBOSOMAL-RNA TRANSCRIPTION TERMINATION IN ANIMAL MITOCHONDRIA

A search of sequence data bases for a tridecamer transcription termina tion signal, previously described in human mtDNA as being responsible for the accumulation of mitochondrial ribosomal RNAs (rRNAs) in excess over the rest of mitochondrial genes, has revealed that this terminat ion signal occurs in equivalent positions in a wide variety of organis ms from protozoa to mammals. Due to the compact organization of the mt DNA, the tridecamer moth usually appears as part of the 3' adjacent ge ne sequence. Because in phylogenetically widely separated organisms th e mitochondrial genome has experienced many rearrangements, it is inte resting that its occurrence near the 3' end of the large rRNA is indep endent of the adjacent gene. The tridecamer sequence has diverged in p hylogenetically widely separated organisms. Nevertheless, a well-conse rved heptamer--TGGCAGA, the mitochondrial rRNA termination box-can be defined. Although extending the experimental evidence of its role as a transcription termination signal in humans will be of great interest, its evolutionary conservation strongly suggests that mitochondrial rR NA transcription termination could be a widely conserved mechanism in animals. Furthermore, the conservation of a homologous tridecamer moth in one of the last 3' secondary loops of nonmitochondrial 23S-like rR NAs suggests that the role of the sequence has changed during mitochon drial evolution.