Mutation of motility genes disrupts normal transcriptional regulation of polyhydroxybutyrate (PHB) cycle genes in Sinorhizobium meliloti

In the nitrogen fixing legume symbiont Sinorhizobium meliloti, intracellular PHB accumulates during unbalanced growth conditions, functioning as carbon and energy store. While the genes for PHB synthesis and degradation are known, the nature of the regulation of PHB accumulation is not defined. To address this, Tn5 mutagenesis was performed on S. meliloti strain Rm1021, and mutants were screened for altered expression of the key synthesis and degradation pathway genes respectively, phbC and bdhA, using lacZ reporter fusions. Each of the expression mutants exhibited phbC overexpression / bdhA underexpression phenotype, and sequence analysis mapped them to motB, visNR and ilvD2 loci. Given that motB and visNR are both involved in synthesis or regulation of flagella, the link to motility was investigated further. Deletion mutants in the motility region of S. meliloti strain RU11 were examined for effects on phbC and bdhA expression. The motB and visR mutants exhibited similar phenotype as corresponding mutants in the Rm1021 background, and they were able to accumulate PHB under conditions that the parental strain is not normally able. Deletion of some other motility genes resulted in unexpected effects on phbC or bdhA expression, some of which have been confirmed by qRT-PCR. While motility does have an effect on PHB cycle regulation, the nature of the regulation mechanism is unknown. Given the role of intracellular PHB as carbon and energy store, it is perhaps not unexpected that accumulation and utilization would be at least in part governed by the quest for available nutrients in the environment.