Ravi Kaul

Interspecies structural diversity among chlamydial genes encoding histone H1

Recently, a eukaryotic histone H1-like protein has been detected in Chlamydia trachomatis serovar L2 [Hackstadt et al., Proc. Natl. Acad. Sci. USA 88 (1991) 3937-3941; Tao et al., J. Bacteriol. 173 (1991) 2818-2822]. We have cloned the corresponding gene from C. trachomatis serovar J and the Chlamydia psittaci strain mn. Sequencing demonstrated absolute gene identity between the two C. trachomatis serovars L2 and J, but divergence in the C. psittaci strain mn. These differences resulted in altered aa residues (in particular no cysteines) and a smaller molecular mass for H1 from C. psittaci strain mn. The amino acid (aa) sequence comparisons with other histone proteins show best alignment to sea urchin H1, notably in the C terminus, for both C. trachomatis and C. psittaci histones. Chlamydial interspecies aa homology, however, is most conserved at the N terminus, suggestive of a bi-functional role for these unique histone proteins.

Cloning and characterization of a secY homolog from Chlamydia trachomatis

Characterization of the genes involved in the process of protein translocation is important in understanding their structure-function relationships. However, little is known about the signals that govern chlamydial gene expression and translocation. We have cloned a 1.7 kb HindIII-PstI fragment containing the secY gene of Chlamydia trachomatis. The complete nucleotide sequence reveals three open reading frames. The amino acid sequence shows highest homology with Escherichia coli proteins L15, SecY and S13, corresponding to the spc-α ribosomal protein operons. The product of the C. trachomatis secY gene is composed of 457 amino acids with a calculated molecular mass of 50 195 Daltons. Its amino acid sequence shows 27.4% and 35.7% identity to E. coli and Bacillus subtilis SecY proteins, respectively. The distribution of hydrophobic amino acids in the C. trachomatis secY gene product is suggestive of it being an integral membrane protein with ten transmembrane segments, the second, third and seventh membrane segments sharing > 45% identity with E. coli SceY. Our results suggest that despite evolutionary differences, eubacteria share a similar protein export apparatus.

Cyclic AMP inhibits protein synthesis in Chlamydia trachomatis at a transcriptional level

Cyclic AMP (cAMP) has an inhibitory effect on the developmental cycle of Chlamydia trachomatis. We examined its influence on the synthesis of chlamydial protein, using the major outer membrane protein (MOMP) as a marker for general chlamydial protein synthesis. During normal development MOMP synthesis accelerates from 18 h post-infection and peaks by 36 h. Cyclic AMP blocks this normal progression of the chlamydial growth cycle. At a concentration of 1 mM, nearly 75% of the total MOMP synthesis was inhibited by 36 h, as monitored by radiolabel uptake. However, no difference was observed during the first 12 h between cAMP-treated and control groups, a finding which is in keeping with correlation between developmental inhibition and protein synthesis. Hybridization studies carried out with a cloned MOMP gene demonstrate a drastic decrease in MOMP mRNA in cAMP-treated cells. Low levels of cAMP utilized in conjunction with a 100,000 x g supernatant from reticulate bodies (RBs) blocked the transcription of the recombinant MOMP gene in an in vitro transcription system. These results suggest that the inhibition of chlamydial protein synthesis, assessed by MOMP synthesis, is due to regulation at a transcriptional level.

Eukaryotic-like histones in Chlamydia.

A fundamental process in all organisms is their ability to regulate gene expression in response to developmental and environmental signals. In Chlamydia, changes in gene expression are closely linked to the presence or to undetectability of eukaryotic-like histones observed late in the parasites life cycle. It is becoming increasingly clear that these histone-like proteins are involved in macromolecular confirmation of DNA. However, their functional role(s) in chlamydial development and the underlying mechanism(s) involved in their degradation and dissociation are largely unknown. It is not surprising therefore that eukaryotic-like histones are a focus of intense research in several laboratories around the world. Recent studies on the interaction of eukaryotic- like histones with DNA, the role of phosphorylation and identification of a histone specific protease are beginning to unravel the mechanism of stage specific differentiation and gene expression in Chlamydia. In this article we review recent advances on the eukaryotic-like histones that have set the stage for elucidation of the chlamydial developmental cycle.