Influence of cysteine deprivation on chlamydial differentiation from reproductive to infective life-cycle forms

The effects of omission of individual amino acids from growth medium on the differentiation of Chlamydia trachomatis DK-20 (serotype E) during infection of cycloheximide-treated McCoy cells are described. As judged by inclusion body staining with acridine orange, omission of cysteine from the medium severely retarded differentiation of reproductive reticulate body (RB) to infective elementary body (EB) forms. The effect appeared specific to cysteine in that omission of other amino acids had little or no effect on differentiation once RBs appeared. On restoration of cysteine, culture infectivity increased and inclusions contained organisms which, by cytochemical and morphological criteria, were differentiating to infective forms, indicating that cysteine deprivation did not irreversibly inhibit differentiation. Impairment of RB to EB differentiation in cysteine-less medium was also observed for three strains of Chlamydia psittaci and 10 other strains of C. trachomatis. It is suggested that the effect arises via the biosynthetic requirement for cysteine for provision of three cysteine-rich proteins, whose synthesis and insertion into the outer membrane have previously been shown to accompany RB to EB differentiation of C. psittaci 6BC and C. trachomatis 434 (serotype L2). Synthesis of cysteine-rich outer membrane proteins during differentiation may thus be common to all chlamydiae.