Salmonella enterica requires ApbC function for growth on tricarballylate: evidence of functional redundancy between ApbC and IscU

Abstract

Mutants of Salmonella enterica lacking apbC have nutritional and biochemical properties indicative of defects in [Fe-S] cluster metabolism. Here we show that apbC is required for S. enterica to use tricarballylate as a carbon and energy source. Tricarballylate catabolism requires three gene products, TcuA, TcuB, and TcuC. Of relevance to this work is the TcuB protein, which has two [4Fe-4S] clusters required for function, making it a logical target for the apbC effect. TcuB activity was 100-fold lower in an apbC mutant than in the isogenic apbC(+) strain. Genetic data show that derepression of the iscRSUA-hscAB-fdx-orf3 operon or overexpression of iscU from a plasmid compensates for the lack of ApbC during growth on tricarballylate. The studies described herein provide evidence that the scaffold protein IscU has a functional overlap with ApbC and that ApbC function is involved in the synthesis of active TcuB.

FIG. 1.

apbC mutants fail to grow on tricarballylate. Strains were grown at 37°C in NCE medium supplemented with thiamine and nicotinic acid and with a sole carbon and energy source. Growth of strains DM10310 (wild type) (○), DM10300 (apbC) (•), DM10325 (iscSUA-hscAB-fdx-orf3) (▴), and DM10667 (iscA-hscAB-fdx-orf3) (□) was monitored on tricarballylate (A) and glucose (B).

FIG. 2.

The overexpression of isc genes allows growth of an apbC mutant on tricarballylate. Strains were grown at 37°C in NCE medium supplemented with thiamine and nicotinic acid and with tricarballylate as a carbon and energy source. (A) A schematic shows the genetic organization of the S. enterica isc operon. The borders of inserts used to generate plasmids are diagrammed below the operon. (B) Growth of strains DM10310 (wild type) (○), JE10435 (iscR11 apbC) (▪), DM10698 (iscR11) (□), DM10474 (apbC iscR7) (▴), and DM10300 (apbC) (•) on tricarballylate NCE medium. (C) Growth of strains DM10310 (wild type) with pSU19 (○), DM10300 (apbC) with pSU19 (•), DM10300 (apbC) with pIscU (▴), and DM10474 (apbC iscR7) with pSU19 (▪) on tricarballylate NCE medium.