DNA sequence and transcript mapping of MOD5: features of the 5' region which suggest two translational starts.

A mutation in the yeast nuclear gene MOD5 drastically reduces the biosynthesis of the modified base isopentenyladenosine in tRNAs located in different cellular compartments: the mitochondria and the nucleus or cytoplasm. Several lines of evidence strongly suggest that MOD5 is the structural gene encoding the tRNA-modifying enzyme delta 2-isopentenyl pyrophosphate:tRNA isopentenyl transferase. DNA sequence analysis of MOD5 reveals an open reading frame of 428 amino acids. A set of mRNAs heterogeneous at both the 5' and 3' termini are transcribed from this gene. Although all of these transcripts initiate upstream of the first AUG codon of the open reading frame, a subset has an extremely short (greater than or equal to 1 base) 5' leader. Furthermore, in positions important for efficient initiation of translation and generally occupied by purines, this first AUG codon is flanked by a U (position -3) and a C (position +4). It is possible that two proteins, one with an amino-terminal extension of basic charge, could be generated from the MOD5 gene via differential translational starts.