초록
Poly-${\beta}$-hydroxybutyrate (PHB) and enzymes related PHB metabolism have been measured in nitrogen-fixing symbiosis of chickpea and cowpea plants. Bacteroids from chickpea and cowpea contained PHB to 0.8% and 43% of their dry weight, respectively, whereas the free-living cells CC 1192 and I 16 produced $285{\pm}55mg$ and $157{\pm}18mg$ of PHB g (dry weight)$^{-1}$. To further understand why chickpea bacteroids contained little PHB, the enzyme activities of PHB metabolism (3-ketothiolase, acetoacetyl-CoA reductase, PHB depolymerase, and 3-hydroxybutyrate dehydrogenase), the TCA cycle (malate dehydrogenase, citrate synthase, and isocitrate dehydrogenase), and related reactions (malic enzyme, pyruvate dehydrogenase, and glutamate:2-oxoglutarate transaminase) were compared in extracts from chickpea and cowpea bacteroids and the respective free-living bacteria. Significant differences were observed between chickpea and cowpea bacteroids and between the bacteroid and free-living forms of CC 1192, with respect to the capacity for some of these reactions. It is indicated that a greater potential for oxidizing malate to oxaloacetate in chickpea bacteroids could be a factor that favors the utilization of acetyl-CoA in TCA cycle rather than for PHB synthesis.