Discovery of intracellular heme-binding protein HrtR, which controls heme efflux by the conserved HrtB-HrtA transporter in Lactococcus lactis

Delphine Lechardeur (1) , Bénédicte Cesselin (1) , Ursula Liebl (2) , Marten H. Vos (2) , Annabelle Fernandez (1) , Celia Brun (1) , Alexandra Gruss (1) , Philippe Gaudu (1)

1 MICALIS - MICrobiologie de l'ALImentation au Service de la Santé
2 LOB - Laboratoire d'optique et biosciences

Most commensal and food bacteria lack heme biosynthesis genes. For several of them, the capture of environmental heme is a means of activating aerobic respiration metabolism. Our previous studies in the Gram-positive bacterium Lactococcus lactis showed that heme exposure strongly induced expression of a single operon, called here hrtRBA, encoding an ortholog of the conserved membrane heme-regulated transporter (hrt) and a unique transcriptional regulator we named HrtR. We show that HrtR expressed as a fusion protein is a heme-binding protein. Heme iron interaction with HrtR is non-covalent, hexacoordinated and involves two histidines, His-72 and His-149. HrtR specifically binds a 15 nt palindromic sequence in the hrtRBA promoter region, which is needed for hrtRBA repression. HrtR-DNA binding is abolished by heme addition, which activates expression of the HrtB-HrtA (HrtBA) transporter in vitro and in vivo. The use of HrtR as an intracellular heme sensor appears to be conserved among numerous commensal bacteria, in contrast with numerous Gram-positive pathogens that use an extracellular heme-sensing system, HssRS, to regulate hrt. Finally, we show for the first time that HrtBA permease controls heme toxicity by its direct and specific efflux. The use of an intracellular heme sensor to control heme efflux constitutes a novel paradigm for bacterial heme homeostasis.

Mots clés: heme; efflux; homeostasis; transcriptional regulator; Lactococcus lactis

Domaines: Sciences du Vivant [q-bio]

Delphine Lechardeur, Bénédicte Cesselin, Ursula Liebl, Marten H. Vos, Annabelle Fernandez, et al.. Discovery of intracellular heme-binding protein HrtR, which controls heme efflux by the conserved HrtB-HrtA transporter in Lactococcus lactis. Cellular and Molecular Life Sciences, 2007, 64 (1), pp.96-103. ⟨10.1074/jbc.M111.297531⟩. ⟨hal-00324331⟩