A Cytoplasmic Male Sterility–Associated Mitochondrial Peptide in Common Bean Is Post-Translationally Regulated

Abstract

Cytoplasmic male sterility in the common bean plant is associated with a dominant mitochondrial mutation designated pvs-or f 239 (for Phaseolus vulgaris sterility sequence open reading frame 239). The sequence is transcribed in both vegetative and reproductive tissues, but the translation product, ORF239, is present only in reproductive tissues. We present evidence to support a model of post-translational regulation of ORF239 expression based on the following observations. In organello translation experiments using purified mitochondria from young seedlings demonstrated accumulation of ORF239 only when a protease inhibitor was included. Proteolytic activity against ORF239 was observed in mitochondrial extracts fractionating with the mitochondrial inner membrane. The DNA sequence encoding a serine-type protease, similar to the lon protease gene of Escherichia coli, was cloned from the Arabidopsis genome. The expression product of this sequence demonstrated proteolytic activity against ORF239 in vitro, with features resembling the activity detected in mitochondrial inner membrane preparations. Antibodies generated against the overexpressed Lon homolog reduced proteolytic activity against ORF239 when added to mitochondrial extracts. Our data suggest that ORF239 was undetected in vegetative tissue due to rapid turnover by at least one mitochondrial protease that acts against ORF239 post-translationally.

Protein stability and degradation in plastids

“…Moreover, immunoblot analysis of purified chloroplasts with an antibody against Lon1 revealed a cross-reacting protein of the correct size that was tightly associated with thylakoid membranes facing the stroma (Ostersetzer et al 2007). This association is consistent with the previous localization of plant Lon to the inner membrane of the mitochondria (Sarria et al 1998), and the archaeal Lon to the plasma membrane (Fukui et al 2002). Expression of Lon4 appears to be constitutive, as its transcript level did not change upon exposure to high light, or low or high temperatures (Sinvany-Villalobo et al 2004).…”

“…It is a hexameric serine protease that uses a Ser-Lys dyad in its active site (Botos et al 2004), which is required for the degradation of abnormal as well as several shortlived regulatory proteins (for review, see Gottesman 1996). Plant homologues of Lon protease (peptidase S16.003) have been identified in mitochondria (Barakat et al 1998;Sarria et al 1998), but apparently they are also found in chloroplasts. Transient-expression assay of GFP fusions revealed that of the four genes found in Arabidopsis, products of Lon1 and Lon2 are targeted to the mitochondria and peroxisomes, respectively, whereas Lon4 is dually targeted to both the mitochondria and chloroplasts (Ostersetzer et al 2007).…”

Alloplasmic male sterility in Brassica napus (CMS 'Tournefortii-Stiewe') is associated with a special gene arrangement around a novel atp9 gene

“…Transcriptional differences between sterile and restored CMS plants result from nucleus-induced changes in the mitochondrial genome (Mackenzie and Chase 1990) or from a nuclear influence on (post-) transcriptional processes (Iwabuchi et al 1993;Moneger et al 1994;Bellaoui et al 1997;Gagliardi and Leaver 1999). It is generally assumed that, upon fertility restoration, either the CMS-specific transcript (Singh and Brown 1991) or the CMS-associated mitochondrial protein (Sarria et al 1998) is quantitatively reduced. In CMS 'Tournefortii-Stiewe' the steady-state level of the 1.58-kb CMS specific transcript is lower in mitochondria from etiolated seedlings than in those from flower buds, and reduced in the fertile offspring of restored plants, supporting the assumption that orf193 (which is only transcribed into this bi-cistronic mRNA) is involved in the expression of the male-sterile phenotype.…”

Mitochondrial Genome Evolution and Gynodioecy

“…Generally, the expression of CMS genes is constitutive, with the exception of CMS protein of Phaseolus vulgaris, which is specifically accumulated in anther tissues (Sarria, Lyznik, Vallejos, & Mackenzie, 1998). This has raised the question of its specificity of action since its expression does not have any pleiotropic effect besides the loss of male fertility, or at least its effects are not too costly (Dufay et al, 2007).…”