Summary: One of the critical developmental events during the unique intracellular life cycle of Chlamydiae is their differentiation from a metabolically active, replicative form or reticulate body (RB) to an infectious extracellular form of the organism (elementary body or EB). This process is characterized by the expression of two extraordinarily cysteine-rich envelope proteins of molecular masses 9 kDa and 60 kDa. We describe the molecular cloning and sequence determination of the 9 kDa cysteine-rich proteins (CrPs) of and . Comparison of these 9 kDa CrP amino acid sequences with those of showed regions of structural variation and conservation. Transcription of the 9 kDa CrP genes occurred as both a monocistronic message and as a bicistronic message which included the 60 kDa CrP gene. Transcription of the 9 kDa and 60 kDa CrP genes was tightly linked to the chlamydial growth cycle with synthesis of their mRNAs and consequent translation of the 60 kDa CrPs occurring as RBs differentiated to form EBs. The maximal rate of transcription occurred late in the growth cycle from a single but highly conserved promoter which had close similarity with the consensus promoter sequences. A stem and loop structure which could be involved in regulating translation of mRNA occurred in all three species between the transcriptional start point and the ribosome binding site. Although transcription is initiated from a single promoter in all three chlamydial species, transcriptional termination points for the monocistronic and bicistronic mRNAs differ in both number and position.


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