Characterization of genes involved in (p)ppGpp precursor biosynthesis in Erwinia amylovora

Previous studies have shown that the hypersensitive response and pathogenicity (hrp)-type III secretion system (T3SS) is activated by the bacterial alarmone (p)ppGpp, which mediates the stringent response in nutrient starvation conditions in Erwinia amylovora. ATP and GTP, the precursors of (p)ppGpp biosynthesis, are synthesized from AMP/adenine and GMP/guanine, respectively, by the de novo pathway of purine biosynthesis. In this study, we examined the role of the purA/purB and guaA/guaB genes, which are involved in the AMP/adenine and GMP/guanine biosynthesis, respectively. Our results showed that growth of the purA, purB, and guaAB mutants was much slower in minimal media, but could be restored by addition of exogenous AMP/adenine and GMP/guanine, respectively. However, mixed results were obtained for their contribution to virulence on immature pear fruits. These results suggested that the de novo purine biosynthesis pathway might be required for proliferation, colonization and infection of E. amylovora.