Rearranged coding segments, separated by a transfer RNA gene, specify the two parts of a discontinuous large subunit ribosomal RNA in Tetrahymena pyriformis mitochondria

Abstract

In the mitochondria of Tetrahymena pyriformis ST, the large subunit ribosomal RNA (LSU rRNA) is discontinuous, consisting of two species, alpha (280 nucleotides in length) and beta (approximately equal to 2.45 kilobases long). LSU alpha is the 5'-terminal portion (i.e. a 5.8 S-like rRNA), and LSU beta constitutes the rest of this LSU rRNA, as judged by both primary and secondary structure homology to other LSU rRNAs. Remarkably, LSU alpha is encoded downstream of LSU beta, the two genes being separated by a tRNALeu gene. This is the first demonstration of a discontinuous rRNA whose coding sequences are rearranged, deviating from the conventional, highly conserved, 5'----3' order of sequence domains in the LSU rRNA gene. This novel gene organization (5'-LSU beta-2-base pair spacer-tRNALeu-10-base pair spacer-LSU alpha) is identical in both copies of the subterminal inverted repeat in the linear T. pyriformis mitochondrial genome. Sequence heterogeneity in the LSU alpha transcript and its genes, together with Northern hybridization data, suggest that both copies of the inverted repeat are expressed.