Ravi Kaul and Wanda M. Wenman

Department of Pediatrics, Section of Infectious Diseases, 403 Neurosciences Building, School of Medicine, 1515 Newton Court, University of California, Davis, CA 95616

Received 2/16/ 98 Accepted 2/20/98

7. PHOSPHORYLATION OF EUKARYOTIC-LIKE HISTONE HC1

DNA decondensation of internalized EB during the early stages of chlamydial infection appears essential for successful initiation of its growth cycle. In vitro studies are supportive of electrostatic interactions between DNA and histone-like proteins that play a paramount role in DNA folding and condensation. The question whether Hc1 is phosphorylated like its eukaryotic counterpart and whether phosphorylated Hc1 exhibits altered affinity for DNA leading to Hc1:DNA dissociation was recently addressed by us (47). Preliminary results are clearly indicative of Hc1 phosphorylation in vivo. While Hc1 phosphorylation peaked around 19 h post infection, Hc1 content increased steadily with the progression of the chlamydial life cycle. Significantly low Hc1 phosphorylation was observed late in the life cycle. Whether this represents dephosphorylation or inability to phosphorylate during later stages of developmental cycle remains to be determined. Nevertheless, these results represent the first report of Hc1 phosphorylation in Chlamydia during early stages of the growth cycle. Further, we were able to phosphorylate both Hc1 and its carboxyl terminus in vitro using protein kinase C (PKC). Recently, we localized a phosphoserine residue at the C-terminal tail of Hc1 as the potential phosphorylation site. Examination of the sequences around the phosphoserine residue identified a motif STKK - a well known PKC phosphorylation site. It remains to be determined if the same site is phosphorylated in vivo and what the source of PKC is. Given the availability of reagents it is possible now to ask whether phosphorylated Hc1 exhibits altered affinity for DNA and if so, whether that is sufficient to weaken the DNA:Hc1 complexes.