Copy number of bean mitochondrial genes estimated by real-time PCR does not correlate with the number of gene loci and transcript levels.

Structural rearrangements characteristic for plant mitochondrial DNA often result in the appearance of genes in new genomic environments. The determination of the real number of gene copies is difficult since the in vivo structure of plant mitochondrial genomes is questionable. It is still uncertain whether the gene copy number regulates transcription in plant mitochondria. Using the real-time PCR technique we have quantified the copies of mitochondrial genes and their transcripts in four related Phaseolus vulgaris lines. We found low intergenomic variation both in the copy number of particular genes and the abundance of their transcripts, while the intragenomic differences between copy numbers and transcripts levels of various genes were much higher. Furthermore, we found that the appearance of a gene in a new location is not correlated with a proportional increase in its copy number estimated by real-time PCR. This observation seems to result from gene dosage compensation which is probably associated with the multimolecular plant mitochondrial genome structure and particularly with the recombinogenic activity of large repeats. Based on the relative gene copy numbers we propose the existence of two types of Phaseolus mitochondrial genomes: one associated with fertility and the other inducing cytoplasmic male sterility. We also show that there is no correlation between the observed number of copies of the analyzed genes and the steady-state level of their transcripts.