Free serine kinase that uses ATP to phosphorylate L-serine to yield O-phospho-L-serine and ADP. O-phospho-L-serine serves as a substrate for SbnA and is a precursor for staphyloferrin B biosynthesis (PubMed:29483190). Is also a DNA-binding regulatory protein that senses heme to control gene expression for siderophore biosynthesis. Binds to DNA within the sbnC coding region and is required for expression of genes in the sbn operon from sbnD onward (PubMed:26534960). ATP + L-serine = ADP + H(+) + O-phospho-L-serine Binds heme and heme binding inhibits DNA binding. kcat is 3.9 min(-1) with ATP as substrate. kcat is 14.3 min(-1) with L-serine as substrate. Siderophore biosynthesis. Forms dimers and tetramers in solution (PubMed:26534960). Predominantly forms dimers (PubMed:29483190). Dimerization/oligomerization is not essential for kinase activity (PubMed:29483190). Up-regulated under iron-deficient growth conditions. Repressed by Fur under iron-rich growth conditions. Mutant is impaired for staphyloferrin B production.