Transcriptional organization, regulation and role of the Porphyromonas gingivalis W83 hmu haemin-uptake locus

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Abstract
Porphyromonas gingivalis, an oral bacterium associated with periodontal disease, requires haemin for growth. Although several multigenic clusters encoding haemin-uptake systems are present on the genome ofP. gingivalis, little is known regarding their transcriptional organization and expression. This study identified a 23 kDa iron-regulated haemin-binding protein encoded by a larger than previously reported variant ofhmuY. It was shown that thehmulocus is larger than previously reported and is composed of six genes,hmuYRSTUV, encoding a novel hybrid haemin-uptake system. The locus has an operonic organization and the transcriptional start site is located 292 bp upstream ofhmuY. The data indicate that the regulation of the operon is iron-dependent. Interestingly, differential regulation within the operon was demonstrated, resulting in excess of thehmuYRmessage encoding the outer-membrane proteins when compared to the full-length transcript. In addition, thehmuYtranscript is more prevalent than thehmuRtranscript. Secondary structure analysis of thehmuYRSTUVmRNA predicted the formation of several potential stem–loops in the 5′ ends ofhmuR- andhmuS-specific mRNAs, consistent with the differential regulation observed. Finally, it was demonstrated that haemin binding and uptake are elevated in iron-depleted conditions and are reduced 45 % and 70 %, respectively, in anhmu-deficient strain when compared to the parental strain, indicating that thehmulocus plays a major role in haemin acquisition inP. gingivalis. Since homologues of thehmulocus were also found inBacteroides fragilis,Bacteroides thetaiotaomicronandPrevotella intermedia, these findings may have implications for a better understanding of haemin acquisition in those organisms as well.

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