The mitochondrial Rieske iron-sulfur protein is an obligatory component of the respiratory electron transport chain that is encoded by a single-copy gene in mammals and fungi. In contrast, this protein is encoded by a small gene family in dicotyledonous tobacco and monocotyledonous maize. We cloned four cDNAs from tobacco that encode the mitochondrial Rieske iron-sulfur protein. These clones, along with a previously isolated cDNA, represent five independent members of the gene family that can be divided into three subfamilies. All of these genes were derived from the two progenitor species and were expressed in amphidiploid tobacco. The proteins encoded by these five genes are probably functional because they all contain the universally conserved hexyl peptides necessary for the 2Fe-2S cluster formation. The expression of the Rieske protein gene family is differentially regulated; a 6- to 11-fold higher level of steady state transcripts was found in flowers than in leaves, stems, and roots. Members of at least two subfamilies were preferentially expressed in flowers, indicating that they share a common cis-regulatory element(s), which can respond to a flower-specific signal(s). Although approximately 10 times more transcripts occurred in flowers than in leaves, flower and leaf mitochondria contained a similar amount of the Rieske protein. Flowers, however, contained seven times more Rieske proteins than leaves. These results indicated an increase in mitochondrion number in flowers. High-energy demands during anther development might bring about an increase in mitochondrion numbers in flowers and the flower-enhanced expression of the Rieske protein gene family. Our results suggested that nuclear genes encoding mitochondrial respiratory proteins could sense and respond to changes in energy metabolism and/or changes in mitochondrion numbers.

The mitochondrial Rieske iron-sulfur protein is an obligatory component of the respiratory electron transport chain that is encoded by a single-copy gene in mammals and fungi. In contrast, this protein is encoded by a small gene family in dicotyledonous tobacco and monocotyledonous maize. We cloned four cDNAs from tobacco that encode the mitochondrial Rieske iron-sulfur protein. These clones, along with a previously isolated cDNA, represent five independent members of the gene family that can be divided into three subfamilies. All of these genes were derived from the two progenitor species and were expressed in amphidiploid tobacco. The proteins encoded by these five genes are probably functional because they all contain the universally conserved hexyl peptides necessary for the 2Fe-2s cluster formation. The expression of the Rieske protein gene family is differentially regulated; a 6-to 11-fold higher leve1 of steady state transcripts was found in flowers than in leaves, stems, and roots. Members of at least two subfamilies were preferentially expressed in flowers, indicating that they share a common cis-regulatory element(s), which can respond to a flowerspecific signal(s). Although -10 times more transcripts occurred in flowers than in leaves, flower and leaf mitochondria contained a similar amount of the Rieske protein. Flowers, however, contained seven times more Rieske proteins than leaves. These results indicated an increase in mitochondrion number in flowers. High-energy demands during anther development might bring about an increase in mitochondrion numbers in flowers and the flower-enhanced expression of the Rieske protein gene family. Our results suggested that nuclear genes encoding mitochondrial respiratory proteins could sense and respond to changes in energy metabolism andlor changes in mitochondrion numbers. INTRODUCTIONThe mitochondrial electron transport chain of eukaryotes consists of four major multimeric enzyme complexes, one of which is the ubiquino1:cytochrome c oxidoreductase, commonly referred to as the cytochrome bcl complex or complex III. Electron flow through the cytochrome bcl complex is coupled with a vectorial transmembrane proton translocation, which generates an electrochemical gradient that is subsequently used for the synthesis of ATP by the Fo/Fl ATPase located in the mitochondrial inner membrane (for review, see Trumpower, 1981). The enzyme complex has been purified from bacteria (Yang and Trumpower, 1986), fungi (Siedow et al., 1978;Weiss and Kolb, 1979), plants (Nakajimaet al., 1984;Berry et al., 1991;Braun and Schmitz, 1992), and mammals (Rieske et al., 1964). Although the number of subunits in the complex varies from as few as three in some bacteria to as many as 11 in some higher eukaryotes, all bcl complexes contain a core of three catalytic subunits, including cytochrome b, which has two heme groups, cytochrome cl, and the Rieske iron-sulfur protein (RISP) that contains a nonheme 2Fe-2s cluster.To whom correspondence should be addressed.The RlSP was first observed in ...