Chlamydial histone homologs control developmental fitness in the next infection cycle

The unique chlamydial developmental cycle comprises three stages: primary differentiation of infectious elementary bodies (EBs) into reticulate bodies (RBs), RB replication, and secondary differentiation into progeny EBs. Extensive chromosome remodeling during RB-to-EB differentiation is thought to be mediated by the histone homologs HctA and HctB. Here, we used an inducible CRISPR interference system to knock down hctA, hctB, or both genes in Chlamydia trachomatis. Surprisingly, knockdown of either histone gene alone or in combination caused only modest reductions in EB yield and did not prevent nucleoid condensation during the parental developmental cycle. In contrast, progeny EBs generated under histone knockdown conditions were markedly impaired in their ability to initiate subsequent infections. Secondary cultures initiated with HctA-deficient EBs exhibited a delay in primary EB-to-RB differentiation, whereas combined repression of hctA and hctB caused both delayed differentiation and persistently reduced genome accumulation, indicative of impaired RB formation and subsequent growth. Repression of hctB alone did not measurably affect genome replication in secondary cultures. Together, these findings reveal that chromosome organization established during EB maturation governs developmental fitness in the next infection cycle, uncovering a transgenerational role for chlamydial histone homologs.IMPORTANCEChlamydial histone homologs HctA and HctB are unusual among bacterial DNA-binding proteins in that they share sequence homology with mammalian histones and are developmentally regulated during the formation of infectious particles. Here, we show that reduced expression of HctA and HctB has only modest effects on visible nucleoid condensation and EB production, consistent with partial functional redundancy between the two histones and suggesting that additional nucleoid factors contribute to EB chromosome compaction. In contrast, deficiency of HctA and HctB during EB maturation has profound consequences in the next infection cycle, impairing completion of primary EB-to-RB differentiation and subsequent RB growth. These findings reveal a previously unrecognized transgenerational role for chlamydial histone homologs and establish chromosome organization during EB maturation as a key determinant of developmental fitness across infection cycles.