The accumulation of PHB in biotin-limited cultures of S. meliloti, but not in biotin-sufficient cultures, suggested that the regulation of a controlling step in the PHB pathway might be influenced by biotin (Hofmann, Heinz et al. 2000). A similar accumulation of PHB by R. etli under biotin-limited conditions had been reported earlier (Encarnación, Dunn et al. 1995). The presence of 4 nM biotin resulted in more than four-fold increase in bdhA-lacZ expression over the level of expression in the absence of biotin (Hofmann, Heinz et al. 2000). This finding is consistent with an enhanced rate of PHB degradation under biotin-sufficient conditions. The location of xdhA2-xdhB2 in the same operon as bdhA means that xanthine dehydrogenase/oxidase activity also probably increases in the presence of biotin. This mixed-function operon arrangement thus links the degradation of intracellular carbon and nitrogen stores to the same biotin-regulated expression.
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