SbnI is a free serine kinase that generates O-phospho-l-serine for staphyloferrin B biosynthesis in Staphylococcus aureus

Staphyloferrin B (SB) is an iron-chelating siderophore produced by Staphylococcus aureus in invasive infections. Proteins for SB biosynthesis and export are encoded by the sbnABCDEF-GHI gene cluster, in which SbnI, a member of the ParB/Srx superfamily, acts as a heme-dependent transcriptional regulator of the sbn locus. However, no structural or functional information about SbnI is available. Here, a crystal structure of SbnI revealed striking structural similarity to an ADP-dependent free serine kinase, SerK, from the archaea Thermococcus kodakarensis. We found that features of the active sites are conserved, and biochemical assays and P-31 NMR and HPLC analyses indicated that SbnI is also a free serine kinase but uses ATP rather than ADP as phosphate donor to generate the SB precursor O-phospho-L-serine (OPS). SbnI consists of two domains, and elevated B-factors in domain II were consistent with the open-close reaction mechanism previously reported for SerK. Mutagenesis of Glu(20) and Asp(58) in SbnI disclosed that they are required for kinase activity. The only known OPS source in bacteria is through the phosphoserine aminotransferase activity of SerC within the serine biosynthesis pathway, and we demonstrate that an S. aureus serC mutant is a serine auxotroph, consistent with a function in L-serine biosynthesis. However, the serC mutant strain could produce SB when provided L-serine, suggesting that SbnI produces OPS for SB biosynthesis in vivo. These findings indicate that besides transcriptionally regulating the sbn locus, SbnI also has an enzymatic role in the SB biosynthetic pathway.