Characterization of the Plesiomonas shigelloides genes encoding the heme iron utilization system

Plesiomonas shigelloides is a gram-negative pathogen which can utilize heme as an iron source. In previous work, P. shigelloides genes which permitted heme iron utilization in a laboratory strain of Escherichia coli were isol ated. In the present study, the cloned P. shigelloides sequences were found to encode ten potential heme utilization proteins: HugA, the putative heme receptor; TonB and ExbBD; HugB, the putative periplasmic binding protein; HugCD, the putative inner membrane permease; and the proteins HugW, HugX, a nd HugZ. Three of the genes, hugA, hugZ, and tonB, contain a Fur box in the ir putative promoters, indicating that the genes may be iron regulated. Whe n the P. shigelloides genes were tested in E. coli K-12 or in a heme iron u tilization mutant of P. shigelloides, hugA, the TonB system genes, and hugW ; hugX, or hugZ were required for heme iron utilization. When the genes wer e tested in a hemA entB mutant off, coli, hugWXZ were not required for util ization of heme as a porphyrin source, but their absence resulted in heme t oxicity when the strains were grown in media containing heme as an iron sou rce. hugA could replace the Vibrio cholerae hutA in a heme iron utilization assay, and V. cholerae hutA could complement a P. shigelloides heme utiliz ation mutant, suggesting that HugA is the heme receptor. Our analyses of th e TonB system of P. shigelloides indicated that it could function in tonB m utants of both E. coli and V. cholerae and that it was similar to the V. ch olerae TonB1 system in the amino acid sequence of the proteins and in the a bility of the system to function in high-salt medium.