Identification and Characterization of Promoters Regulating tuf Expression in Chlamydia trachomatis Serovar F

Gene expression in the obligate intracellular bacterium Chlamydia trachomatis ranges from nil in the infectious EB form to high in the dividing RB form. Little is known about the mechanisms of gene regulation in chlamydiae and only a few promoter sequences have been characterized. The purpose of our study was to examine the expression of a cluster of genes that are required for translation in C. trachomatis serovar F: infA (encoding Initiation Factor 1), tRNAThr, tuf (encoding Elongation Factor Tu), and tRNATrp. Primer extension analysis indicated that tuf is expressed in three different mRNAs. Putative promoter sequences for these transcripts were defined as P1 (upstream of tRNAThr), P2 (within infA) and P3 (upstream of infA). Quantitative RT-PCR analysis revealed that P1 transcripts were most abundant at 16 h postinfection (pi), whereas P2 transcripts predominated at 24 h pi. P3 was active at all times pi; however, transcription terminated upstream of tuf at early times pi and continued through tuf at later times. P1 and P3 were active in Escherichia coli, as assessed by CAT expression in promoter-fusion vectors and a chlamydial in vitro transcription system. Site-specific mutagenesis confirmed the importance of the −35 and −10 hexamers in the P1 and P3 promoters. P2 was weakly active in E. coli and inactive in the in vitro transcription system, indicating either that the P2 transcript is processed from a longer transcript or that P2 expression requires a sigma or transcription factor which is not present in E. coli or the in vitro transcription system. Our data suggest that multiple processes play a role in the regulation of tuf gene expression during the developmental cycle.