Paramecium mitochondrial genes. II. Large subunit rRNA gene sequence and microevolution.

Mature Paramecium mitochondrial large subunit rRNA consists of two stable segments: a 20 S segment described previously and a unique 283-base segment similar to 5.8 S rRNAs typically found in eucaryotic cytoplasmic RNA. pBR325 clones of both gene regions from both Paramecium primaurelia and Paramecium tetraurelia were sequenced and aligned. The gene segments lie adjacent to each other very near the replicative terminal end of the linear Paramecium mitochondrial genome and are transcribed from a common 23 S precursor. The precise gene ends were determined using nuclease S1 protection; the large subunit rRNA gene complex (consisting of "5.8 S-like" rRNA, a 19-26-base excised region, and 20 S rRNA) spans about 2654 base pairs. The gene complex is preceded by a 15-base poly(T) tract and terminates randomly within a 20-base A + T-rich segment immediately preceding the tRNATyr gene. The sequences from the two species were 4% divergent, the changes consisting of 59% transitions, 38% transversions, and 3% insertions or deletions. The sequences were aligned with Escherichia coli 23 S rRNA, and a secondary structure model is presented for the entire molecule based on structures proposed for E. coli 23 S rRNA.