Purine Biosynthesis, Biofilm Formation, and Persistence of an Insect-Microbe Gut Symbiosis
ABSTRACTTheRiptortus-Burkholderiasymbiotic system is an experimental model system for studying the molecular mechanisms of an insect-microbe gut symbiosis. When the symbiotic midgut ofRiptortus pedestriswas investigated by light and transmission electron microscopy, the lumens of the midgut crypts that harbor colonizingBurkholderiasymbionts were occupied by an extracellular matrix consisting of polysaccharides. This observation prompted us to search for symbiont genes involved in the induction of biofilm formation and to examine whether the biofilms are necessary for the symbiont to establish a successful symbiotic association with the host. To answer these questions, we focused onpurNandpurT, which independently catalyze the same step of bacterial purine biosynthesis. When we disruptedpurNandpurTin theBurkholderiasymbiont, the ΔpurNand ΔpurTmutants grew normally, and only the ΔpurTmutant failed to form biofilms. Notably, the ΔpurTmutant exhibited a significantly lower level of cyclic-di-GMP (c-di-GMP) than the wild type and the ΔpurNmutant, suggesting involvement of the secondary messenger c-di-GMP in the defect of biofilm formation in the ΔpurTmutant, which might operate via impaired purine biosynthesis. The host insects infected with the ΔpurTmutant exhibited a lower infection density, slower growth, and lighter body weight than the host insects infected with the wild type and the ΔpurNmutant. These results show that the function ofpurTof the gut symbiont is important for the persistence of the insect gut symbiont, suggesting the intricate biological relevance of purine biosynthesis, biofilm formation, and symbiosis.