Loss of the mRNA-like region in mitochondrial tmRNAs of jakobids.

It has been postulated that a highly reduced form of transfer messenger RNA (tmRNA), a bacterial molecule involved in the rescue of stalled ribosomes during translation, is expressed in the mitochondrion of the jakobid Reclinomonas americana. Here we show that genes encoding both one-piece and two-piece tmRNAs are present in six different jakobid mitochondrial DNAs. Mitochondrial tmRNAs have retained the highly conserved tRNA(Ala)-like domain, but they apparently lack the mRNA-like region present in all bacterial tmRNAs. Comparative analysis of jakobid mitochondrial genomes shows that a potential mRNA-like region in R. americana (orf64) is located at distant genomic positions in other jakobids. Our results strongly suggest that orf64 is a tatA homolog. Through Northern hybridization we confirm the postulated reduced size of both a one-piece tmRNA in Jakoba libera and a two-piece tmRNA in Seculamonas ecuadoriensis. The J. libera tmRNA is post-transcriptionally modified by addition of a 3' CCA tail, processed in vitro by RNase P RNA, and specifically charged with alanine in vitro by alanyl-tRNA synthetase. Our results strongly support the functionality of these reduced mitochondrial tmRNAs.