Isolation and characterization of cDNA for a plant mitochondrial phosphate translocator (Mpt1): ozone stress induces Mpt1 mRNA accumulation in birch (Betula pendula Roth).

We have isolated by DDRT-PCR (differential-display reverse-transcription polymerase chain reaction) and cDNA library screening a 1.3 kb cDNA corresponding to a strongly ozone-inducible transcript from birch (Betula pendula Roth). Nucleotide sequence analysis suggests that it encodes a mitochondrial phosphate translocator protein (Pic), the first one isolated from plants. The isolated birch mitochondrial phosphate translocator cDNA (designated Mpt1) contains an open reading frame of 1092 bases encoding a 364 amino acid polypeptide. The deduced protein is 66% similar to bovine Pic isoform B. Comparison of the N-terminal amino acid sequence to known mammalian Pic proteins and the existence of an in-frame stop codon upstream of the initiation codon suggest that the isolated cDNA is full-length. Southern hybridization analysis of birch genomic DNA shows that Mpt1 is a single-copy gene. Accumulation of Mpt1 mRNA during oxidative stress imposed by ozone is detectable already at 2 h and it is at maximum ca. 12 h after the beginning of an 8 h ozone exposure (150 ppb). A second O3 peak at 48-56 h did not increase transcript levels further. O3 exposure for 2 h was sufficient for Mpt1 induction. Birch Mpt1 transcript levels remain at moderately low level during leaf development and is lower in roots and leaves when compared to young shoots undergoing wood formation and lignification.