Porphyromonas gingivalis HmuY and HmuR: further characterization of a novel mechanism of heme utilization
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Porphyromonas gingivalis HmuY is a putative heme-binding lipoprotein associated with the outer membrane. It is part of an operon together with a gene encoding an outer-membrane hemin utilization receptor (HmuR) and four uncharacterized genes. A similar operon organization was found in Bacteroides fragilis and B. thetaiotaomicron, with the former containing an additional HmuY homologue encoded upstream of the hmuR-like gene. In P. gingivalis cultured under heme-limited conditions, a ∼1-kb hmuY transcript was produced at high levels along with some ∼3.5 and ∼9-kb transcripts. Compared with the parental strain, mutants deficient in hmuY or hmuR or hmuY–hmuR gene function grew more slowly and bound lower amounts of hemin and hemoglobin. Significantly, they grew more slowly or were unable to grow when human serum was used as the sole iron/heme source. Analysis of the hmu promoter showed that it is regulated by iron. The HmuY protein normally occurs as a homodimer, but in the presence of hemin it may form tetramers. These results show that HmuY may be the first reported member of a new class of proteins in Porphyromonas and Bacteroides species involved in heme utilization, a function being exerted in conjunction with HmuR, an outer-membrane heme transporter.
KeywordsPorphyromonas gingivalis HmuR Heme outer-membrane receptor HmuY Heme-binding protein Heme uptake Lipoprotein His-tag
Dr. C.A. Genco (Section of infectious diseases, Boston University School of Medicine, USA) is gratefully acknowledged for giving T.O. the opportunity to continue studies on P. gingivalis heme utilization. The authors thank Dr. Klaus Hantke (University of Tübingen, Germany) for generously supplying us with the E. coli K-12 EB53 strain. This work was supported in part by grants nos. 3 P05A 113 24 and N401 029 32/0742 from the Department of Scientific Research, Ministry of Science and Higher Education, Poland (T.O.) and NIH grant (DE 09761), USA (J.P.). J.P. is a Subsydium Profesorskie award recipient from the Foundation for Polish Science (Warsaw, Poland). Part of the data was presented at the FEBS Congress and the 9th IUBMB Conference (Budapest, Olczak et al. 2005, FEBS J 272S1:15).
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