Genetic and Molecular Basis of Kingella kingae Encapsulation
Kingella kingaeis a common cause of invasive disease in young children and was recently found to produce a polysaccharide capsule containingN-acetylgalactosamine (GalNAc) and β-3-deoxy-d-manno-octulosonic acid (βKdo). Given the role of capsules as important virulence factors and effective vaccine antigens, we set out to determine the genetic determinants ofK. kingaeencapsulation. Using a transposon library and a screen for nonencapsulated mutants, we identified the previously identifiedctrABCD(ABC transporter) operon, alipA(kpsC)-like gene, alipB(kpsS)-like gene, and a putative glycosyltransferase gene designatedcsaA(capsulesynthesis typeageneA). These genes were found to be present at unlinked locations scattered throughout the genome, an atypical genetic arrangement for Gram-negative bacteria that elaborate a capsule dependent on an ABC-type transporter for surface localization. ThecsaAgene product contains a predicted glycosyltransferase domain with structural homology to GalNAc transferases and a predicted capsule synthesis domain with structural homology to Kdo transferases, raising the possibility that this enzyme is responsible for alternately linking GalNAc to βKdo and βKdo to GalNAc. Consistent with this conclusion, mutation of the DXD motif in the GalNAc transferase domain and of the HP motif in the Kdo transferase domain resulted in a loss of encapsulation. Examination of intracellular and surface-associated capsule in deletion mutants and complemented strains further implicated thelipA(kpsC)-like gene, thelipB(kpsS)-like gene, and thecsaAgene inK. kingaecapsule production. These data define the genetic requirements for encapsulation inK. kingaeand demonstrate an atypical organization of capsule synthesis, assembly, and export genes.