Cysteine desulphurase-encoding gene sufS2 is required for the repressor function of RirA and oxidative resistance in Agrobacterium tumefaciens

Abstract

The Agrobacterium tumefaciens genome contains a cluster of genes that are predicted to encode Fe–S cluster assembly proteins, and this cluster is known as the sufS2BCDS1XA operon. sufS2 is the first gene in the operon, and it was inactivated to determine its physiological function. The sufS2 mutant exhibited a small colony phenotype, grew slower than the wild-type strain and was more sensitive to various oxidants including peroxide, organic hydroperoxide and superoxide. The sufS2 gene was negatively regulated by iron response regulator (Irr) and rhizobial iron regulator (RirA) under low and high iron conditions, respectively, and was inducible in response to oxidative stress. The oxidant-induced expression of sufS2 was controlled by Irr, RirA and an additional but not yet identified mechanism. sufS2 was required for RirA activity in the repression of a sufS2 promoter-lacZ fusion. RirA may use Fe–S as its cofactor. sufS2 disruption may cause a defect in the Fe–S supply and could thereby affect the RirA activity. The three conserved cysteine residues (C91, C99 and C105) in RirA were predicted to coordinate with the Fe–S cluster and were shown to be essential for RirA repression of the sufS2-lacZ fusion. These results suggested that sufS2 is important for the survival of A. tumefaciens.