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

The Chlamydia trachomatis histone H1-like protein (Hc1) is a DNA bindi ng protein specific for the metabolically inactive chlamydial developm ental form, the elementary body. Hcl induces DNA condensation in Esche richia coli and is a strong inhibitor of transcription and translation , These effects may, in part, be due to Hc1-mediated alterations of DN A topology, To locate putative functional domains within Hcl, polypept ides 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-his (succinic acid N-hydroxysuccinimide ester), purified recombinant Hcl was found to form dimers, The dimerization si te was located in the N-terminal part of Hc1 (Hc1(2-57)). Moreover, ci rcular dichroism measurements indicated an overall alpha-helical struc ture of this region, By using limited proteolysis, Southwestern blotti ng, and gel retardation assays, Hc1(53-125) was shown to contain a dom ain capable of binding both DNA and RNA. Under the same conditions, Hc 1(2-57) had no nucleic acid-binding activity. Electron microscopy of H c1-DNA and Hc1(53-125)-DNA complexes revealed differences suggesting t hat the N-terminal part of Hc1 may affect the DNA-binding properties o f Hc1.