We identified and sequenced a segment of Chlamydia trachomatis chromosomal DNA that shows homology to the Escherichia coli spc and distal region of the S10 ribosomal protein (r-protein) operons. Its sequence revealed a high degree of nucleotide and operon context conservation with the E. coli r-protein genes. The C. trachomatis spc operon contains the r-protein genes for L14, L24, L5, S8, L6, L18, S5, L15, and Sec Y along with the genes for r-proteins L16, L29, and S17 of the S10 operon. The two operons are separated by a 16-bp intragenic region which contains no transcription signals. However, a putative promoter for the transcription of the spc operon was found 162 nucleotides upstream of the CtrL14e start site; it revealed significant homology to the E. coli consensus promoter sequences. Interestingly, our results indicate the absence of any structure resembling an EcoS8 regulatory target site on C. trachomatis spc mRNA in spite of significant amino acid identity between E. coli and C. trachomatis r-proteins. Also, the intrinsic aminoglycoside resistance in C. trachomatis is unlikely to be mediated by CtrL6e since E. coli expressing CtrL6e remained susceptible to gentamicin (MIC < 0.5 ,ug/ml).Chlamydia trachomatis infections represent major public health problems in both developing and industrialized countries (30). Chlamydia species have evolved a complex and unique developmental cycle which involves two distinct forms: the small (0.2 to 0.3 ,um) extracellular, rigid elementary bodies and the large (1 ,um) intracellular, fragile reticulate bodies (44). The genetics of chlamydial regulation is largely undefined, mainly because of the lack of any convenient system for gene transfer and also because of the paucity of information about the signals and machinery that govern gene expression.Ribosomes constitute the protein-synthesizing machinery of both prokaryotes and eukaryotes. The entire prokaryotic ribosome comprises three rRNAs with sedimentation coefficients of 23S, 16S, and 5S and approximately 52 ribosomal proteins (r-proteins) that are organized into 19 different operons (26,33). Earlier attempts at characterizing the rRNA from Chlamydia species have met with some success. Tamura and Iwanaga (42) identified 21S, 16S, and 4S rRNA fractions in Chlamydia psittaci; of these, 21S and 16S were more predominant forms in reticulate bodies, whereas 4S predominated in the elementary bodies. Sarov and Becker found similar rRNA species in C. trachomatis (39). On the basis of their 16S rRNA gene sequence, chlamydiae have been identified as eubacterial in origin, related peripherally to planctomyces (45). Although a few other ribosomal determinants have been identified, r-protein gene organization and expression have not been well characterized (7-10, 20, 21). Interestingly, the organization and transcriptional regulation of r-protein operons appear to be well conserved among eubacteria; however, their regulation among evolutionary distant species remains unelucidated (15).Recently, we reported the cloning and sequen...