Synthesis of protein by the obligate intracellular parasitic bacteria Chlamydia psittaci (6BC) and Chiamydia trachomatis (serovar L2) isolated from host cells (host-free chlaniydiae) was demonstrated for the first time. Incorporation of [35S]methionine and [35S]cysteine into trichloroacetic acid-precipitable material by reticulate bodies of chlamydiae persisted for 2 h and was dependent upon a exogenous source of ATP, an ATP-regenerating system, and potassium or sodium ions. Magnesium ions and

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... mino acids stimulated synthesis; chloramphenicol, rifampin, oligomycin, and carbonyl cyanide p-trifluoromethoxyphenylhydrazone (a proton ionophore) inhibited incorporation. Ribonucleoside triphosphates (other than ATP) had little stimulatory effect. The optimum pH for host-free synthesis was between 7.0 and 7.5. The molecular weights of proteins synthesized by host-free reticulate bodies closely resembled the molecular weights of proteins synthesized by reticulate bodies in an intracellular environment, and included outer membrane proteins. Elementary bodies of chlamydiae were unable to synthesize protein even when incubated in the presence of 10 mM dithiothreitol, a reducing agent which converted the highly disulfide bond cross-linked major outer membrane protein to monomeric form.