The 5′-leader sequence of sugar beet mitochondrial atp6 encodes a novel polypeptide that is characteristic of Owen cytoplasmic male sterility

Abstract

Cytoplasmic male sterility (CMS) is a mitochondrially encoded trait, which is characterized by a failure of plants to produce viable pollen. We have investigated the protein profile of mitochondria from sugar beet plants with normal (fertile) or CMS cytoplasm, and observed that a 35-kDa polypeptide is expressed in Owen CMS plants but not in normal plants. The variant 35-kDa polypeptide was found in CMS mitochondria placed in five different nuclear backgrounds. Interestingly, this polypeptide proved to be antigenically related to a 387-codon ORF (preSatp6) that is fused in-frame with the downstream atp6. The presequence extension of the atp6 ORF is commonly found in higher plants, but whether or not it is normally expressed has hitherto remained unclear. Our study is thus the first to demonstrate that the atp6 presequence is actually translated in mitochondria. We also observed that preSATP6 is a mitochondrial membrane protein that assembles into a homogeneous 200-kDa protein complex. In organello translation experiments in the presence of protease inhibitors showed a reduction in the abundance of mature preSATP6 with time, suggesting that the mature preSATP6 may be derived by proteolytic processing of a translation product of the preSatp6/Satp6 ORF.

Cost of Having the Largest Mitochondrial Genome: Evolutionary Mechanism of Plant Mitochondrial Genome

“…Two ORFs in sugar beet mitochondria, orf119 (consisting of a duplicated segment of nad9 and 340 bp of unknown origin) and orf324 (containing a duplicated segment of atp8 and 751 bp of unknown origin), are transcribed along with the downstream genes atp1 and rps13, respectively. However, the ORF119 and ORF324 antisera failed to react with any mitochondrial proteins, and no band corresponding to such polypeptides was observed among the radiolabelled in organello translation products [81][82][83]. A possible explanation is that the unique transcripts lack cis regulatory elements required for translation.…”

“…This motif is the target site of an endopeptidase, presumably the orthologue of the yeast ATP23 [91,92] (see below). There is no evidence that the cleaved N-terminal extension is accumulated as a solitary polypeptide, apart from one exception: sugar beet atp6, whose N-terminal extension is associated with CMS and known as preSatp6 [83]. In cereal mitochondria, the gene rps2 encodes a fused C-terminal extension [93] that is not encoded in the Marchantia, Physcomitrella, or Cycas mitochondria.…”

“…It is not known whether there are any roles for these extensions in angiosperm mitochondria, or whether the fusion proteins have acquired novel functions, apart from the exception of the sugar beet atp6 [83]. No extensions have been found in mammalian atp6 genes; however an N-terminal extension (10 amino acid residues) of atp6 is required for the efficient assembly of the mature ATP6 polypeptide into the ATP9 ring in Saccharomyces cerevisiae [94].…”

“…A variant of atp6 in sugar beet termed preS-3atp6, whose encoded amino acid sequence shows 88% homology with that of preSatp6, is translated and the corresponding polypeptide is detected in leaves and roots but not anthers. This suggests that the polypeptide may be accessible to a tissue-specific degradation system, unlike preSATP6, which is quite stable [83,101].…”

Mitochondrial DNA Phylogeny of Cultivated and Wild Beets: Relationships Among Cytoplasmic Male-Sterility-Inducing and Nonsterilizing Cytoplasms

“…The protein proved to be the product of a 387-codon mitochondrial ORF (designated preSatp6), which is fused in frame with the downstream atp6 ORF (Yamamoto et al 2005). Interestingly, this protein was not detectable in the anther tissue of two different sources of CMS ½I-12CMS(2) and I-12CMS (3) from wild Beta beets collected in Turkey and Pakistan; instead, a CMS-associated protein of 12 kDa was identified (Hallden et al 1992; M. P. Yamamoto, Y. Onodera, T. Kubo and T. Mikami, unpublished results).…”