Haemophore‐mediated signal transduction across the bacterial cell envelope in Serratia marcescens: the inducer and the transported substrate are different molecules

Abstract

Summary: Numerous bacteria are able to use free and haemoprotein‐bound haem as iron sources because of the action of small secreted proteins called haemophores. Haemophores have very high affinity for haem, and can therefore extract haem from the haem‐carrier proteins and deliver it to the cells by means of specific cell surface receptors. Haem is then taken up and the empty haemophores are recycled. Here, we report a study of the regulation of the Serratia marcescens has operon which is involved in haemophore‐dependent haem acquisition. We characterized two genes encoding proteins homologous to specific ECF sigma and antisigma factors. We showed that they regulate the synthesis of the haemophore‐specific outer membrane receptor, HasR, by a signal transduction mechanism similar to the siderophore surface‐signalling systems. We also showed the essential role of HasR itself in this process. Using haem‐loaded and haem‐free haemophore, we identified the stimulus for the HasR‐mediated signal transduction as being the binding of the haem‐loaded haemophore to HasR. Thus, unlike siderophore‐uptake systems, in which the signalling molecule is the transported substrate itself, in the haemophore‐dependent haem uptake system the inducer and the transported substrate are different compounds.