The mitochondrial genome of a cytoplasmic male sterile line of perennial ryegrass (Lolium perenne L.) contains an integrated linear plasmid-like element

Abstract

The mitochondrial genome of a cytoplasmic male sterile line of perennial ryegrass (Lolium perenne L.) was shown to contain a 9.6 kb element, LpCMSi, that is absent in the mitochondrial genome of fertile lines. LpCMSi contains the previously described chimeric gene orfC9, and three additional open reading frames (orfs) encoding a unique 45 kDa predicted protein of unknown function, a family B-like DNA polymerase (LpDpo), and a phage-type single subunit RNA polymerase (LpRpo). The latter two proteins shared significant similarity with DNA and RNA polymerases encoded by extrachromosomal linear mitochondrial plasmids of plants and fungi, and also to integrated plasmid-like sequences found in various plant and fungal mitochondrial genomes. Transcripts for both LpDpo and LpRpo were detected by RT-PCR in mitochondria of the CMS line. PCR-based investigations further revealed the presence of LpCMSi-like sequences in fertile L. perenne lines that are likely maintained as low-copy number extrachromosomal replicons. The absence of integrated forms of LpCMSi in the mitochondrial genome of fertile lines suggests that LpCMSi integration adjacent to the atp9 gene may be responsible, directly or indirectly, for the sterility phenotype of the CMS line.