Two DNA-binding proteins with similarity to eukaryotic histone H1 have been described in Chlamydia trachomatis. In addition to the 18-kDa histone H1 homolog Hc1, elementary bodies of C. trachomatis possess an antigenically related histone H1 homolog, which we have termed Hc2, that varies in apparent molecular mass among strains. We report the molecular cloning, expression, and nucleotide sequence of the hctB gene encoding Hc2 and present evidence for in vivo DNA-binding activity of the expressed product. Expression of Hc2 in Escherichia coli induces a compaction of bacterial chromatin that is distinct from that observed upon Hcl expression. Moreover, isolated nucleoids from Hc2-expressing E. coli exhibit markedly reduced sensitivity to DNase 1. These properties of Hc2 are consistent with a postulated role in establishing the nucleoid structure of elementary bodies.

Chlamydia trachomatis elementary bodies contain two developmentally expressed histone H1 homologues. An 18-kDa histone homologue, Hc1, is conserved among C. trachomatis serovars and C. psittaci. The other histone homologue, Hc2 (encoded by hctB), varies in size between C. trachomatis serovars but is present in reduced amounts or absent from C. psittaci. The variation in Hc2 size among C. trachomatis serovars was found to be due to internal deletions from a region of the hctB gene encoding lysine- and alanine-rich pentameric repeats.

Chlamydiae are obligate intracellular bacteria which undergo a unique developmental cycle, alternating between non-replicative elementary bodies (EBs) and replicative reticulate bodies (RBs). The transition from RB to EB is characterized by condensation of the chromosome into a dense nucleoid structure. The chlamydial histone homologue Hc1 is sufficient to induce formation of a similar structure in Escherichia coli. High-level Hc1 expression in E. coli is self-limiting and down-regulates transcription, translation, and replication at concentrations similar to those observed in chlamydial elementary bodies. Expression of Hc1 at sub-structural levels may have specific regulatory functions through its interaction with chromosomal DNA. In E. coli this is reflected in a dramatic shift in the pattern of gene expression. The differential expression of the outer membrane porin proteins OmpC and OmpF and analysis of lacZ fusions with promoter regions sensitive to supercoiling suggests that low-level Hc1 expression results in a net relaxation of chromosomal DNA. Topological analysis of plasmid DNA from both E. coli and Chlamydia trachomatis supports a decrease in superhelicity preceding nucleoid formation. In vitro analysis of purified Hc1-DNA interactions supports preferential binding based upon DNA conformation. These results suggest a dual role in which Hc1-mediated changes in gene expression may precede metabolic inactivity.