Porphyromonas gingivalis HmuY and HmuR: further characterization of a novel mechanism of heme utilization

Abstract

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 approximately 1-kb hmuY transcript was produced at high levels along with some approximately 3.5 and approximately 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.

The nucleoid-associated protein HUβ affects global gene expression in Porphyromonas gingivalis

“…The molecular mechanisms underlying iron uptake have been extensively studied (Olczak et al, 2005). Genes involved in iron acquisition include the hmuYRSTUV locus (PG1551-PG1556), which encodes a novel hybrid haemin-uptake system (Brunner et al, 2010;Lewis et al, 2006;Olczak et al, 2005Olczak et al, , 2008Simpson et al, 2000Simpson et al, , 2004Smalley et al, 2011); the hbp35 gene (PG0616), which encodes a haemin-binding protein that also exhibits thioredoxin activity (Hiratsuka et al, 2010;Shoji et al, 2010); the surface protein HusA (PG2227), which acts as a haemophore with very high affinity (Gao et al, 2010); the tlr (PG0644) iron transport locus Slakeski et al, 2000); the haemin-binding protein FetB (PG0669) and the surrounding genes in that locus; and PG0465, the ferric uptake regulator (fur) (Olczak et al, 2005). In addition, regulatory mechanisms controlling expression of iron acquisition genes have been identified, including the transcriptional activator encoded by PG1237, which has been shown to control expression of the hmu locus (Wu et al, 2009).…”

Regulation of Hemin Binding Proteins by a Novel Transcriptional Activator in Porphyromonas gingivalis

“…Iron concentration is a common regulatory signal that usually elicits expression modulation of genes associated with iron acquisition. Previous studies have shown that P. gingivalis hmu genes are upregulated in iron-depleted or -limited conditions (10,16), suggesting the presence of an iron-dependent repression of hmu genes in P. gingivalis. In the present study, we also observed an upregulation of hmuY expression in P. gingivalis grown in hemin-limited conditions.…”

“…It is well documented that expression of hmu genes is under tight control in P. gingivalis (8,10,16). However, the mechanism responsible for this regulation of hmu gene expression is unknown.…”

“…Since expression of genes involved in iron uptake systems is known to be selectively regulated by the iron concentration in the microenvironment (11), several previous studies focused on the regulation of hmu expression in hemin-limited conditions (10,16). These studies showed an upregulation of hmuY and hmuR expression in P. gingivalis grown under iron-depleted or -limited conditions.…”

“…It is reported that under hemin-limited conditions, P. gingivalis elevates expression of hmuY and hmuR (10,15,16,18). However, the mechanism of regulation of the expression of iron uptake genes is not well established for P. gingivalis.…”