Identification of a 5-nucleotide sequence that controls expression of the ica locus in Staphylococcus aureus and characterization of the DNA-binding properties of IcaR



Biofilm formation is an important aspect of the pathogenesis of staphylococcal infections. A β-1,6-linked N-acetyl glucosamine polysaccharide is critical to biofilm elaboration and is synthesized by proteins encoded by the intercellular adhesion (ica) locus. These studies were undertaken to characterize the mechanism by which transcription of the ica locus in S. aureus is regulated using isogenic S. aureus MN8 and MN8 mucoid (MN8m) strains, the latter of which constitutively overproduces biofilm. Transformation of the ica locus from MN8m to the ica knock-out mutants of two strains, MN8 and NCTC 10833, conferred a strong biofilm-producing phenotype. Sequence analysis revealed a 5-nucleotide deletion within the promoter region of the ica locus in MN8m compared with the sequence in the wild-type locus. Deletion or substitution of these 5 nucleotides within the wild-type ica locus augmented transcription of the ica locus and induced the strong biofilm-producing phenotype. Gel shift analysis demonstrated that a protein(s) within cell-free lysates from strain MN8 bind(s) specifically to oligonucleotides representative of the wild-type ica promoter sequence and that this binding is greatly diminished by the deletion or substitution of the 5 nucleotides. DNase I footprint analysis revealed that purified IcaR, thought to be a regulator of ica transcription, also binds to the ica promoter sequence just upstream of the ica start codon, but its affinity for the ica promoter is unaffected by deletion of the 5-nucleotide motif. These findings identify a 5-nucleotide motif within the ica promoter region that has a functional role in transcriptional regulation of the ica locus that is independent of IcaR, and also show that IcaR binds to the promoter region of the S. aureus ica locus.