Plesiomonas shigelloides hugZ encodes an iron-regulated heme binding protein required for heme iron utilization

Plesiomonas shigelloides is an intestinal pathogen that uses heme as an iron source. The P. shigelloides heme utilization system consists of 10 genes, 7 of which permit heme transport and 3 of which are associated with utilization of heme as an iron source once it is inside the cell. The goal of this study was to examine hugZ, 1 of the 3 genes associated with utilization of heme iron. DPH8, a hugZ mutant, failed to grow to full cell density in media containing heme as the iron source, indicating that hugZ is required for heme iron utilization. Western blots using antibodies against Vibrio cholerae HutZ to detect the P. shigelloides HugZ indicated that hugZ encodes an iron-regulated cytoplasmic protein, which is absent in DPH8. A heme affinity bead assay performed on soluble protein fractions from P. shigelloides DPH8/pHUG24.5 (pHUG24.5 encodes hugZ) indicated that HugZ binds heme. Heme utilization was restored in DPH8 by hox1, which encodes the alpha-heme oxygenase from Synechocystis sp. strain PCC6803. However, HugZ did not exhibit alpha-heme oxygenase activity in an assay that detects the conversion of heme to the bilin functional group present in phycobiliproteins. These results do not rule out that HugZ exhibits another type of heme oxygenase activity not detected in the assay.