Phase Variation of Poly-N-Acetylglucosamine Expression in Staphylococcus aureus

Abstract

Polysaccharide intercellular adhesin (PIA), also known as poly-N-acetyl-β-(1–6)-glucosamine (PIA/PNAG) is an important component of Staphylococcus aureus biofilms and also contributes to resistance to phagocytosis. The proteins IcaA, IcaD, IcaB, and IcaC are encoded within the intercellular adhesin (ica) operon and synthesize PIA/PNAG. We discovered a mechanism of phase variation in PIA/PNAG expression that appears to involve slipped-strand mispairing. The process is reversible and RecA-independent, and involves the expansion and contraction of a simple tetranucleotide tandem repeat within icaC. Inactivation of IcaC results in a PIA/PNAG-negative phenotype. A PIA/PNAG-hyperproducing strain gained a fitness advantage in vitro following the icaC mutation and loss of PIA/PNAG production. The mutation was also detected in two clinical isolates, suggesting that under certain conditions, loss of PIA/PNAG production may be advantageous during infection. There was also a survival advantage for an icaC-negative strain harboring intact icaADB genes relative to an isogenic icaADBC deletion mutant. Together, these results suggest that inactivation of icaC is a mode of phase variation for PIA/PNAG expression, that high-level production of PIA/PNAG carries a fitness cost, and that icaADB may contribute to bacterial fitness, by an unknown mechanism, in the absence of an intact icaC gene and PIA/PNAG production.

Unusual structures of TTTA repeats in icaC gene of Staphylococcus aureus

“…During DNA replication, if this mini-loop occurs transiently in the nascent strand (Fig. 4A), this will lead to one repeat expansion, thus providing a possible explanation for the additional TTTA repeat observed in the icaC gene of S. aureus variants JB12 and NRS264 [8]. Meanwhile, our NMR results also reveal that TTTA repeats are capable to fold into the higher order dumbbell structure with only two repeats.…”

“…1) [8]. The additional repeat shifts the reading frame of icaC, resulting in a premature stop codon and thus a shorter protein.…”

“…Besides, two other S. aureus clinical isolates, namely, NRS264 and NRS63, also show the PIA/PNAG-negative phenotype and their icaC genes have been found to contain one and two additional TTTA repeats, respectively ( Fig. 1) [8].…”

New insights into the genetic instability in CCTG repeats

“…The resulting mini-loop intermediate provides a possible explanation for the observed one-repeat expansion in S. aureus mutants [28]. Yet, this mini-dumbbell exchange was not observed in the NMR studies of the sequence containing three CCTG repeats [10], probably due to the unfolding and refolding of the first and third repeats were hindered by the interactions between these two repeats in the slipped hairpin structure (Fig.…”

Biofilm: An Important Bacterial Feature Still to Deal With

“…According to the literature, the accumulation phase seems to be dependent on adhesins and exopolysaccharides that promote adhesive interaction among bacterial cells [55].The staphylococcal polysaccharide intercellular adhesin called exopolysaccharide (PIA) or poly N -acetilglucosamine (PNAG) encoded by single operon, is an important component of the staphylococcal biofilm extracellular matrix [56].The partial deacetylation of this molecule produces positively charged residues that interact with the matrix component [57]. In addition, the group of surface proteins called MSCRAMMs (microbial surface components recognizing adhesive matrix molecules) is also major determinants of the initial bacterial attachment on tissues and biomaterials.…”