Identification of the nik gene cluster of Brucella suis: Regulation and contribution to urease activity

Analysis of a Brucella suis 1330 gene fused to a gfp reporter, and identifi ed as being induced in J774 murine macrophage-like cells, allowed the isola tion of a gene homologous to nikA, the first gene of the Escherichia coli o peron encoding the specific transport system for nickel. DNA sequence analy sis of the corresponding B. suis nik locus showed that it was highly simila r to that of E. coli except for localization of the nikR regulatory gene, w hich lies upstream from the structural nikABCDE genes and in the opposite o rientation. Protein sequence comparisons suggested that the deduced nikABCD E gene products belong to a periplasmic binding protein-dependent transport system. The nikA promoter-gfp fusion was activated in vitro by low oxygen tension and metal ion deficiency and was repressed by NiCl2 excess. Inserti onal inactivation of nikA strongly reduced the activity of the nickel metal loenzyme urease, which was restored by addition of a nickel excess. Moreove r, the nikA mutant of B. suis was functionally complemented with the E. col i nik gene cluster, leading to the recovery of urease activity. Reciprocall y, an E. coli strain harboring a deleted nik operon recovered hydrogenase a ctivity by heterologous complementation with the B. suis nik locus. Taking into account these results, we propose that the nik locus of B. suis encode s a nickel transport system, The results further suggest that nickel could enter B. suis via other transport systems. Intracellular growth rates of th e B, suis wild-type and nikA mutant strains in human monocytes were similar , indicating that nikA was not essential for this step of infection. We dis cuss a Possible role of nickel transport in maintaining enzymatic activitie s which could be crucial for survival of the bacteria under the environment al conditions encountered within the host.