The <named-content content-type="genus-species">Enterococcus faecalis</named-content> EbpA Pilus Protein: Attenuation of Expression, Biofilm Formation, and Adherence to Fibrinogen Start with the Rare Initiation Codon ATT

Abstract

ABSTRACT The endocarditis and biofilm-associated pili (Ebp) are important in Enterococcus faecalis pathogenesis, and the pilus tip, EbpA, has been shown to play a major role in pilus biogenesis, biofilm formation, and experimental infections. Based on in silico analyses, we previously predicted that ATT is the EbpA translational start codon, not the ATG codon, 120 bp downstream of ATT, which is annotated as the translational start. ATT is rarely used to initiate protein synthesis, leading to our hypothesis that this codon participates in translational regulation of Ebp production. To investigate this possibility, site-directed mutagenesis was used to introduce consecutive stop codons in place of two lysines at positions 5 and 6 from the ATT, to replace the ATT codon in situ with ATG, and then to revert this ATG to ATT; translational fusions of ebpA to lacZ were also constructed to investigate the effect of these start codons on translation. Our results showed that the annotated ATG does not start translation of EbpA, implicating ATT as the start codon; moreover, the presence of ATT, compared to the engineered ATG, resulted in significantly decreased EbpA surface display, attenuated biofilm, and reduced adherence to fibrinogen. Corroborating these findings, the translational fusion with the native ATT as the initiation codon showed significantly decreased expression of β-galactosidase compared to the construct with ATG in place of ATT. Thus, these results demonstrate that the rare initiation codon of EbpA negatively regulates EbpA surface display and negatively affects Ebp-associated functions, including biofilm and adherence to fibrinogen. IMPORTANCE Enterococcus faecalis is among the leading causes of serious infections in the hospital setting, and the endocarditis and biofilm-associated pili (Ebp) have been shown to play significant roles in E. faecalis pathogenesis. Understanding the regulation of virulence is important for the development of new approaches to counteract multidrug-resistant pathogens. We previously predicted that ATT, which has been reported to start protein synthesis only in rare instances, is the most likely translational start codon of EbpA in E. faecalis. Here, we demonstrate that ATT is the initiation codon of EbpA and, relative to a constructed ATG start codon, results in smaller amounts of EbpA on the surface of the cells, attenuating biofilm formation and fibrinogen adherence, phenotypes associated with the ability of E. faecalis to cause infections. This provides the first example of pilus regulation through the use of an ATT initiation codon.