Chlamydia trachomatis Oligopeptide Transporter Performs Dual Functions of Oligopeptide Transport and Peptidoglycan Recycling

Abstract

Peptidoglycan, the sugar-amino acid polymer that composes the bacterial cell wall, requires a significant expenditure of energy to synthesize and is highly immunogenic. To minimize the loss of an energetically expensive metabolite and avoid host detection, bacteria often recycle their peptidoglycan, transporting its components back into the cytoplasm, where they can be used for subsequent rounds of new synthesis. The peptidoglycan-recycling substrate binding protein (SBP) MppA, which is responsible for recycling peptidoglycan fragments in <i>Escherichia coli</i>, has not been annotated for most intracellular pathogens. One such pathogen, <i>Chlamydia trachomatis</i>, has a limited capacity to synthesize amino acids <i>de novo</i> and therefore must obtain oligopeptides from its host cell for growth. Bioinformatics analysis suggests that the putative <i>C. trachomatis</i> oligopeptide transporter OppABCDF (OppABCDF <i>_Ct</i> ) encodes multiple SBPs (OppA1 <i>_Ct</i> , OppA2 <i>_Ct</i> , and OppA3 <i>_Ct</i> ). Intracellular pathogens often encode multiple SBPs, while only one, OppA, is encoded in the <i>E. coli</i><i>opp</i> operon. We hypothesized that the putative OppABCDF transporter of <i>C. trachomatis</i> functions in both oligopeptide transport and peptidoglycan recycling. We coexpressed the putative SBP genes (<i>oppA1_Ct</i> , <i>oppA2_Ct</i> , <i>oppA3_Ct</i> ) along with <i>oppBCDF_Ct</i> in an <i>E. coli</i> mutant lacking the Opp transporter and determined that all three chlamydial OppA subunits supported oligopeptide transport. We also demonstrated the <i>in vivo</i> functionality of the chlamydial Opp transporter in <i>C. trachomatis</i> Importantly, we found that one chlamydial SBP, OppA3 <i>_Ct</i> , possessed dual substrate recognition properties and is capable of transporting peptidoglycan fragments (tri-diaminopimelic acid) in <i>E. coli</i> and in <i>C. trachomatis</i> These findings suggest that <i>Chlamydia</i> evolved an oligopeptide transporter to facilitate the acquisition of oligopeptides for growth while simultaneously reducing the accumulation of immunostimulatory peptidoglycan fragments in the host cell cytosol. The latter property reflects bacterial pathoadaptation that dampens the host innate immune response to <i>Chlamydia</i> infection.