In another example of diverse BCAT function, a Nicotiana benthamiana chloroplastic BCAT was implicated in transcriptional regulation of KNOX genes that affect levels of gibberellins. Two studies in Arabidopsis showed that both the chloroplastic AtBCAT3 and the cytosolic AtBCAT4 also participate in Met chain elongation and the production of aliphatic glucosinolates ( Schuster et al., 2006 Knill et al., 2008). Expression of the other AtBCATs is not as tissue specific ( Liepman and Olsen, 2004). AtBCAT2 expression is observed only in flowers and is elevated under stress, while AtBCAT6 is expressed in flowers and siliques. AtBCAT1 catabolizes all BCAAs in almost all tissue types, and its affinity is greatest in the order Ile > Leu > Val. AtBCAT1 is the most likely candidate for initiating BCAA breakdown ( Schuster and Binder, 2005), although AtBCAT5 has also been found in mitochondrial fractions ( Binder et al., 2007). Complementation analysis in BCAT-deficient yeast strains confirmed the functions of AtBCAT1, -2, -3, -5, and -6 but not AtBCAT4 ( Diebold et al., 2002). AtBCAT4 is cytosolic ( Schuster et al., 2006), and the location of AtBCAT6 is suggested to be cytosolic because of its lack of a defined target peptide sequence ( Diebold et al., 2002). AtBCAT2, -3, and -5 localize to chloroplasts, suggesting roles in BCAA synthesis ( Diebold et al., 2002). AtBCAT1 localizes to mitochondria and is thought to be active primarily in catabolism. In spinach ( Spinacia oleracea), there are two known BCATs, one with a higher affinity toward KIV and the other with a higher affinity toward KIC and KMV, indicating substrate preference ( Binder et al., 2007). BCATs have been studied in only a few plant species. Leu is converted to 4-methyl-2-oxopentanoic acid (KIC), Ile to 3-methyl-2-oxopentanoic acid (KMV), and Val to 3-methyl-2-oxobutanoic acid (KIV). (2010) showed that a mutation in isovaleryl-CoA dehydrogenase, an enzyme in the BCAA catabolic pathway, influences the metabolism of many unrelated compounds in Arabidopsis ( Arabidopsis thaliana) seeds, including 12 amino acids.īranched-chain aminotransferase (BCAT) enzymes are at the interface of BCAA synthesis and catabolism, reversibly catalyzing the interconversion of BCAAs to BCKAs. BCAA catabolism likely has other functions in plant metabolism. The primary fates of BCAAs in plant cells are peptide elongation, glutamate recycling, Glc- and Suc-linked branched-chain esters, branched-chain fatty acid synthesis, and respiration through the synthesis of tricarboxylic acid cycle intermediates ( Kandra et al., 1990 Walters and Steffens, 1990 Kroumova et al., 1994 Daschner et al., 1999 Li et al., 2003 Beck et al., 2004 Taylor et al., 2004 Engqvist et al., 2009). Catabolism is believed to be initiated in mitochondria, where the branched-chain keto acid (BCKA) dehydrogenase complex is located ( Taylor et al., 2004). 8, Isopropylmalate dehydrogenase.Īlthough synthesis of BCAAs is well characterized in plants, regulation of catabolism is not completely understood. Together, these results support a model in which the mitochondrial SlBCAT1 and -2 function in BCAA catabolism while the chloroplastic SlBCAT3 and -4 function in BCAA synthesis. Conversely, antisense-mediated reduction of SlBCAT1 resulted in higher levels of BCAAs. While overexpression of SlBCAT1 or -3 in tomato fruit did not significantly alter amino acid levels, an expression quantitative trait locus on chromosome 3, associated with substantially higher expression of Solanum pennellii BCAT4, did significantly increase BCAA levels. SlBCAT3 and -4 exhibited a preference for the forward reaction, while SlBCAT1 and -2 were more active in the reverse reaction. All enzymes were active in the forward (BCAA synthesis) and reverse (branched-chain keto acid synthesis) reactions. SlBCAT1, -2, -3, and -4 were able to restore growth of Escherichia coli BCAA auxotrophic cells, but SlBCAT1 and -2 were less effective than SlBCAT3 and -4 in growth restoration. SlBCAT1 and -2 are located in the mitochondria, SlBCAT3 and -4 are located in chloroplasts, while SlBCAT5 and -6 are located in the cytosol and vacuole, respectively. SlBCAT1, -2, -3, and - 4 are expressed in multiple plant tissues, while SlBCAT5 and - 6 were undetectable. In this study, six BCAT genes from the cultivated tomato ( Solanum lycopersicum) were identified and characterized. The interface of BCAA metabolism lies with branched-chain aminotransferases (BCAT) that catalyze both the last anabolic step and the first catabolic step. Branched-chain amino acids (BCAAs) are synthesized in plants from branched-chain keto acids, but their metabolism is not completely understood.
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