A DNA-POLYMERASE-RELATED READING FRAME (POL-R) IN THE MTDNA OF SECALE-CEREALE

Mitochondrial (mt)DNA of Secale cereale contains an open reading frame (pol-r), the potential translation product of which shows significant homology to the type-B DNA polymerase encoded by the Si plasmid of Ze a mays; it contains the highly-conserved domains IIa to V of family B polymerases. The pol-r ORF is transcribed, as proven by RT-PCR, but th e transcript is not edited. Upstream of the putative start codon a pot ential promoter motif was detected, fitting well into the postulated c onsensus sequence of the transcription initiation regions of Z. mays a nd Triticum aestivum. The pol-r ORF occurs in mtDNA of the fertile rye variety ''Halo'' and the cytoplasmic male-sterile (CMS) line ''Pampa' '. Both ORFs are almost identical, apart from the 3' terminus; pol-r f rom Halo can code for 289 amino acids, pol-r from Pampa for 312 amino acids. Based on codon usage and the lack of editing, pol-r is consider ed to be a ''young'' gene, probably introduced in the mtDNA of rye by recombination with an mt plasmid.