J. Bacteriol., Feb 1996, 994-1002, Vol 178, No. 4
Copyright © 1996, American Society for Microbiology

The 18-kilodalton Chlamydia trachomatis histone H1-like protein (Hc1) contains a potential N-terminal dimerization site and a C-terminal nucleic acid-binding domain

LB Pedersen, S Birkelund, A Holm, S Ostergaard and G Christiansen
Department of Medical Microbiology and Immunology, University of Aarhus, Denmark.

The Chlamydia trachomatis histone H1-like protein (Hc1) is a DNA- binding protein specific for the metabolically inactive chlamydial developmental form, the elementary body. Hc1 induces DNA condensation in Escherichia coli and is a strong inhibitor of transcription and translation. These effects may, in part, be due to Hc1-mediated alterations of DNA topology. To locate putative functional domains within Hc1, polypeptides Hc1(2-57) and Hc1(53-125), corresponding to the N- and C-terminal parts of Hc1, respectively, were generated. By chemical cross-linking with ethylene glycol-bis (succinic acid N- hydroxysuccinimide ester), purified recombinant Hc1 was found to form dimers. The dimerization site was located in the N-terminal part of Hc1 (Hc1(2-57)). Moreover, circular dichroism measurements indicated an overall alpha-helical structure of this region. By using limited proteolysis, Southwestern blotting, and gel retardation assays, Hc1(53- 125) was shown to contain a domain capable of binding both DNA and RNA. Under the same conditions, Hc1(2-57) had no nucleic acid-binding activity. Electron microscopy of Hc1-DNA and Hc1(53-125)-DNA complexes revealed differences suggesting that the N-terminal part of Hc1 may affect the DNA-binding properties of Hc1.