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

4. IDENTIFICATION OF EUKARYOTIC-LIKE HISTONE GENES

The application of molecular cloning techniques over the past decade has provided insight into the regulation of a few gene products including histone H1-like proteins that appear to play a major role in controlling the chlamydial developmental cycle through their ability to modify DNA structure. In 1991, two groups working independently reported the cloning and characterization of a very basic protein with an estimated pI of 10.71 (18, 19). The highly basic nature of this polypeptide probably explained the discrepancy between its estimated (18,000 daltons) and calculated (13,689 daltons) molecular weights. Protein data base searches identified significant homology between the cloned gene product and eukaryotic histone H1. This protein is now commonly termed Hc1. Monoclonal antibody generated against chlamydial Hc1 displayed immunoblot cross reactivity to Hc1 and a related EB specific protein with an apparent molecular mass of 32kDa. These antibodies also revealed antinuclear specificity suggestive of cross reactivity to histone H1 (19). Late stage specific expression of Hc1 was confirmed by Northern blot analysis as well as by immunoblot analysis. Hc1 specific transcript was detected 12h post infection among Chlamydia infected host cells. All C. trachomatis serovars revealed complete conservation of the Hc1 gene product. However, interspecies structural diversity resulted in a smaller protein with 117 amino acids for C. psittaci as compared to a product of 125 amino acids for the C. trachomatis serovars. Greater preservation of amino acid residues was observed among 66 amino terminal residues (87% identity; 20). In contrast, Hc2 exhibited variable molecular weights of 25-32 kDa depending upon the serovar. Perara et al (21) identified the gene encoding the 26-kDa Hc2 from C. trachomatis serovar MoPn. Examination of the derived amino acid sequence identified pentapeptide motifs containing three aliphatic residues (usually valine or alanine) and two basic residues (lysine and arginine) which appear crucial for DNA binding. Recently, the Hc2 gene encoding the 32 kDa protein from C. trachomatis serovar L₂ has been cloned and expressed by Brickman et al (22). This Hc2 polypeptide chain also contains numerous pentapeptide repeats of three aliphatic and two basic residues. It has been suggested that the range of molecular weights observed in different serovars is due to truncated forms of a common protein, since they all share amino acid sequence in the amino terminus. Considering the sequence variability of Hc2 from different C. trachomatis serovars and its absence in C. psittaci strain Meningopneumonitis (Mn), the functional importance of Hc2 in vivo remains to be elucidated (23). It must be emphasized that in general all prokaryotic cells synthesize a set of small, usually basic proteins collectively called histone-like proteins (because their biochemical properties resemble eukaryotic histones) that bind and compact DNA (24). However, unlike chlamydial histone H1-like protein Hc1, which shows nearly 74% similarity (considering 38 perfectly matched residues and 45 conservative substitutions) to eukaryotic histone H1, these prokaryotic nucleoproteins exhibit very little sequence homology. Eukaryotic-like histones have been described in two other prokaryotes, Pseudomonas aeruginosa (25), and Bordetella pertussis (26). In P. aeruginosa histone H1-like protein AlgR3 binds directly to a specific DNA sequence leading to positive regulation of exopolysaccharide alginate biosynthesis (27) while in Bordetella pertusis histone-like protein BpH1 plays a role in chromatin formation and condensation (26).