ORFB is a subunit of F₁ F_O‐ATP synthase: insight into the basis of cytoplasmic male sterility in sunflower

Abstract

ORFB is the product of a gene that is conserved in plant mitochondrial genomes, and which, on the basis of sequence motif and structural similarity, is predicted to be the homologue of yeast and mammalian ATP8, part of the F O component of the F 1 F O -ATP synthase. We have shown that, in sunflower, orfB transcripts are edited, increasing the similarity of the predicted protein to ATP8 proteins from non-plant species. Blue-native polyacrylamide gel electrophoresis and peptide sequencing confirm that ORFB localizes to the ATP synthase complex. The predicted amino-terminal 19 amino acids of ORFB are identical to those in the chimeric mitochondrial ORF522 protein, which is associated with cytoplasmic male sterility (CMS) in sunflower. Assays comparing respiratory complexes from a male-sterile line expressing ORF522 with those from a male-fertile line show a specific decrease in ATP hydrolysis by the ATP synthase. These observations allow us to propose a mechanism underlying CMS that is associated with the expression of chimeric open reading frames containing part of the orfB gene.

Magnesium transporter AtMGT9 is essential for pollen development in Arabidopsis

“…As Mg 2+ is a critical cofactor of ATP synthases and other enzymes in the respiration processes, Mg 2+ level should serve as an essential regulatory element of mitochondrial activity. As mitochondrial defects have long been implicated in male sterility, it is not fortuitous that AtMGT5 plays an essential role in pollen development [26][27][28][29]. Although due to technical difficulty the sub-cellular localization of AtMGT9 protein has not been determined, it stands as a possibility that this transporter may also be localized in the mitochondria and regulate Mg 2+ homeostasis in this organelle.…”

Interactions of Mitochondrial and Nuclear Genes That Affect Male Gametophyte Development

“…A 159-codon sunflower gene has been shown to encode ATPase subunit 8 (Sabar et al, 2003). Sequences highly similar to this gene are found in many other plant mitochondrial genomes and have been termed orfB in previous reports.…”

“…Following this lead, the region around atpA was characterized further, leading to the discovery of a novel ORF that encodes 522 nucleotides (174 codons) downstream of atpA Laver et al, 1991). The first 19 amino acids of ORF522 are identical to those of ATP8 (Sabar et al, 2003). In restored lines, transcripts of the novel orf gene are reduced in florets but not in seedlings (Moneger et al, 1994).…”

The Rf and Rf-like PPR in higher plants, a fast-evolving subclass of PPR genes

“…[4][5][6] A few CMS-causing proteins were effectively shown to moderately impair respiratory chain activity or to form large pores within the mitochondrial inner membrane, but how this correlates with pollen abortion remains elusive. [7][8][9][10][11] Male fertility can be restored by the expression of nuclearencoded genes called restorer of fertility (Rf ), which re-establish partial to normal pollen production to plants carrying a corresponding sterility-inducing cytoplasm. Fertility restoration is generally associated with a strong reduction in the production of mitochondrial CMS-inducing proteins.…”