Pii: S0378-1119(00)00504-7

It has been proposed that splice-variants of proteins involved in mitochondrial RNA processing and translation may be involved in the tissue speci®city of mitochondrial DNA disease mutations (Fischel-Ghodsian, 1998. Mol. Genet. Metab. 65, 97±104). To identify and characterize the structural components of mitochondrial RNA processing and translation, the Mammalian Mitochondrial Ribosomal Consortium has been formed. The 338 amino acid (aa) residues long MRP-L5 was identi®ed (O'Brien et al., 1999. J. Biol. Chem. 274, 36043± 36051), and its transcript was screened for tissue speci®c splice-variants. Screening of the EST databases revealed a single putative splicevariant, due to the insertion of an exon consisting of 89 nucleotides prior to the last exon. Screening of multiple cDNA libraries revealed this inserted exon to be present only in heart tissue, in addition to the predominant MRP-L5 transcript. Sequencing of this region con®rmed the EST sequence, and showed in the splice-variant a termination triplet at the beginning of the last exon. Thus the inserted exon replaces the coding sequence of the regular last exon, and creates a new 353 aa long protein (MRP-L5V1). Sequence analysis and 3D modeling reveal similarity between MRP-L5 and threonyl-t-RNA synthetases, and a likely RNA binding site within MRP-L5, with the C-terminus in proximity to the RNA binding site. Sequence analysis of MRP-L5V1 also suggests a likely transmembrane domain at the C-terminus. Thus it is possible that the MRP-L5V1 C-terminus could interfere with RNA binding and may have gained a transmembrane domain. Further studies will be required to elucidate the functional signi®cance of MRP-L5V1. q 2000 Elsevier Science B.V. All rights reserved.