The strategy that we have used, the isolation of mutants unable to grow using PHB cycle intermediates as sole carbon source, has allowed us to identify the genes involved in the degradation part of the PHB cycle. The genetics of the system is leading us towards a more complete understanding of the role of the PHB cycle in the soil, rhizosphere and during nodulation. The linkage of the metabolism of PHB carbon stores and intracellular purine stores by the bdhA-xdhA2-xdhB2 mixed function operon suggests a model for the colonization of the alfalfa root. In the rhizosphere, high levels of available carbon source coupled with the relatively lower levels of nitrogen would result in PHB accumulation. The nitrogen limitation limits growth, leading to rRNA degradation, and some of the purines from the degraded rRNA would be accumulated intracellularly as hypoxanthine. Thus, cells in the rhizosphere would contain PHB and hypoxanthine stores. Biotin in the alfalfa rhizosphere would then induce transcription of bdhA-xdhA2-xdhB2 operon, resulting in degradation of both PHB and hypoxanthine stores and concomitantly increased cell growth and colonization of the alfalfa root. This model remains to be tested experimentally.
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