Beyond Tryptophan Synthase: Identification of Genes That Contribute to Chlamydia trachomatis Survival during Gamma Interferon-Induced Persistence and Reactivation

Abstract

cChlamydia trachomatis can enter a viable but nonculturable state in vitro termed persistence. A common feature of C. trachomatis persistence models is that reticulate bodies fail to divide and make few infectious progeny until the persistence-inducing stressor is removed. One model of persistence that has relevance to human disease involves tryptophan limitation mediated by the host enzyme indoleamine 2,3-dioxygenase, which converts L-tryptophan to N-formylkynurenine. Genital C. trachomatis strains can counter tryptophan limitation because they encode a tryptophan-synthesizing enzyme. Tryptophan synthase is the only enzyme that has been confirmed to play a role in interferon gamma (IFN-␥)-induced persistence, although profound changes in chlamydial physiology and gene expression occur in the presence of persistence-inducing stressors. Thus, we screened a population of mutagenized C. trachomatis strains for mutants that failed to reactivate from IFN-␥-induced persistence. Six mutants were identified, and the mutations linked to the persistence phenotype in three of these were successfully mapped. One mutant had a missense mutation in tryptophan synthase; however, this mutant behaved differently from previously described synthase null mutants. Two hypothetical genes of unknown function, ctl0225 and ctl0694, were also identified and may be involved in amino acid transport and DNA damage repair, respectively. Our results indicate that C. trachomatis utilizes functionally diverse genes to mediate survival during and reactivation from persistence in HeLa cells. Chlamydia trachomatis has a characteristic biphasic developmental cycle in cell culture (1). The two dominant chlamydial forms are elementary bodies (EBs) and reticulate bodies (RBs) (1). EBs represent the infectious, nonreplicative form capable of invading host cells (1). Following entry, EBs differentiate into RBs that replicate inside parasitophorous vacuoles termed inclusions (1). Maturing RBs then redifferentiate back to EBs and exit the host cell via lysis or extrusion (2). Morphologically aberrant RBs have been observed in clinical specimens (3). This abnormal developmental cycle can be recapitulated in a laboratory setting by exposing C. trachomatis to various stressors such as host cytokines (4-6), excess amino acids (7), iron deficiency (8, 9), virus coinfections (10), and antibiotics (11, 12) (reviewed in reference 13). Under these conditions, normal C. trachomatis RBs can transition into persistent forms that do not divide, are enlarged, and have aberrant morphology (13). Upon removal of the persistence-inducing stressor, RBs transition back into normal RBs and continue normal development (13). Aberrant RB morphology is a shared feature of multiple C. trachomatis persistence models and may reflect activation of a common stress response program (13). Thus, characterizing how C. trachomatis enters, survives, and reactivates from persistence could reveal new insights into chlamydial pathogenesis.The Th1 cytokine interferon gamma (IFN-␥) can induce dif...

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