ISOLATION AND CHARACTERIZATION OF CDNA FOR A PLANT MITOCHONDRIAL PHOSPHATE TRANSLOCATOR (MPT1) - OZONE STRESS INDUCES MPT1 MESSENGER-RNA ACCUMULATION IN BIRCH (BETULA-PENDULA ROTH)

We have isolated by DDRT-PCR (differential-display reverse-transcripti on polymerase chain reaction) and cDNA library screening a 1.3 kb cDNA corresponding to a strongly ozone-inducible transcript from birch (Be tula pendula Roth). Nucleotide sequence analysis suggests that it enco des a mitochondrial phosphate translocator protein (P(i)c), the first one isolated from plants. The isolated birch mitochondrial phosphate t ranslocator cDNA (designated Mpt1) contains an open reading frame of 1 092 bases encoding a 364 amino acid polypeptide. The deduced protein i s 66% similar to bovine P(i)c isoform B. Comparison of the N-terminal amino acid sequence to known mammalian P(i)c proteins and the existenc e of an in-frame stop codon upstream of the initiation codon suggest t hat the isolated cDNA is full-length. Southern hybridization analysis of birch genomic DNA shows that Mpt1 is a single-copy gene. Accumulati on 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 a n 8 h ozone exposure (150 ppb). A second O-3 peak at 48-56 h did not i ncrease transcript levels further. O-3 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 wh en compared to young shoots undergoing wood formation and lignificatio n.