The hmuUV genes of Sinorhizobium meliloti 2011 encode the permease and ATPase components of an ABC transport system for the utilization of both haem and the hydroxamate siderophores, ferrichrome and ferrioxamine B.

Sinorhizobium meliloti, the endosymbiont of Medicago sativa, can use haem compounds, including haemoglobin and leghaemoglobin, when growing in the free-living state. The components of the system involved in haem acquisition were confirmed to be ShmR, an outer membrane receptor, and HmuTUV, predicted to be an ABC transport system comprising a periplasmic protein, a permease and an ATPase respectively. The roles of HmuTUV in haem transport were confirmed in a heterologous expression system in Escherichia coli in conjunction with HasR, the outer membrane haem receptor of Serratia marcescens. hmuTUV mutants of S. meliloti showed a reduced capacity to acquire haem, suggesting the presence of a second haem acquisition system in the organism. S. meliloti can also acquire iron from xenosiderophores and the genes encoding the outer membrane receptors for ferrichrome and ferrioxamine B, fhuA1 and fhuA2, respectively, were identified. In light of this it is proposed that fhuA2 should be renamed foxA in the S. meliloti 1021 genome sequence. A siderophore reductase, FhuF, with the capacity to complement an E. coli ferrioxamine B reductase mutant, was identified encoded by a gene next to fhuA2. In the same transcriptional unit as fhuF the gene fhuP was identified and shown to encode a protein necessary for transport of ferrichrome and ferrioxamine B and predicted to be periplasmic. Interestingly, the remaining components of the transport system for the siderophores are HmuU and HmuV. Ferrichrome, ferrioxamine B and haem compounds therefore share components of the same transport system in S. meliloti.