Fidelity of Translation Initiation of mRNA for the Precursor of Rat Mitochondrial Serine:Pyruvate/Alanine:Glyoxylate Aminotransferase1

Abstract

Serine:pyruvate/alanine:glyoxylate aminotransferase (SPT/AGT) of rat liver is localized in both mitochondria and peroxisomes. The rat SPT/AGT gene is single, but there are two species of mRNA which differ at their 5' termini due to transcription from two alternative initiation sites. The longer mRNA is translated from the first AUG codon and thereby directs synthesis of the 45 kDa precursor of mitochondrial SPT/AGT, which includes a mitochondria-targeting N-terminal signal sequence. Peroxisomal SPT/AGT is synthesized as a product of mature size (43 kDa) from the shorter mRNA, which starts 3' to the first AUG codon and thus is translated from a downstream AUG codon. In our previous immunocytochemical study, SPT/AGT was found to be localized only in peroxisomes, when a cDNA encoding 43 kDa SPT/AGT was expressed in COS cells. When a cDNA encoding the 45 kDa precursor was expressed, on the other hand, SPT/AGT was localized mostly in mitochondria, but a small number of peroxisomes were also positively stained [Yokota, S., Funai, T., and Ichiyama, A. (1991) Biomed. Res. 12, 53-59]. We show in this paper that 43 kDa SPT/AGT is also synthesized from the longer mRNA in an in vitro translation system through a leaky scanning mechanism. Although the first AUG initiator codon is in a suboptimal context, the amount of 43 kDa SPT/AGT synthesized from the longer mRNA was small, probably because a downstream stem-loop structure facilitates recognition of the first AUG initiator codon.