PhaR, a Negative Regulator of PhaP, Modulates the Colonization of a Burkholderia Gut Symbiont in the Midgut of the Host Insect, Riptortus pedestris

Five genes encoding PhaP family proteins and one gene have been identified in the genome of symbiont strain RPE75. PhaP proteins function as the surface proteins of polyhydroxyalkanoate (PHA) granules, and the PhaR protein acts as a negative regulator of PhaP biosynthesis. Recently, we characterized one gene to understand the molecular cross talk between insects and gut symbionts. In this study, we constructed four other gene-depleted mutants (Δ , Δ , Δ , and Δ mutants), one gene-depleted mutant, and a -complemented mutant (Δ mutant). To address the biological roles of four family genes and the gene during insect-gut symbiont interaction, these mutants were fed to the second-instar nymphs, and colonization ability and fitness parameters were examined. , the Δ and Δ mutants cannot make a PHA granule normally in a stressful environment. Furthermore, the Δ mutation decreased the colonization ability in the host midgut and negatively affected the host insect's fitness compared with wild-type -infected insects. However, other family gene-depleted mutants colonized well in the midgut of the fifth-instar nymph insects. However, in the case of females, the colonization rate of the Δ mutant was decreased and the host's fitness parameters were decreased compared with the wild-type-infected host, suggesting that the environment of the female midgut may be more hostile than that of the male midgut. These results demonstrate that PhaR plays an important role in the biosynthesis of PHA granules and that it is significantly related to the colonization of the gut symbiont in the host insects' midgut. Bacterial polyhydroxyalkanoate (PHA) biosynthesis is a complex process requiring several enzymes. The biological roles of PHA granule synthesis enzymes and the surface proteins of PHA granules during host-gut symbiont interactions are not fully understood. Here, we report the effects on colonization ability in the host midguts and the fitness of host insects after feeding mutant cells (four -depleted mutants and one -depleted mutant) to the host insects. Analyses of both synthesized PHA granule amounts and CFU numbers suggest that the gene is closely related to synthesis of the PHA granule and the colonization of the gut symbiont in the host insect's midgut. Like our previous report, this study also supports the idea that the environment of the host midgut may not be favorable to symbiotic cells and that PHA granules may be required to adapt in the host midgut.