The gyrB transcription start site and promoter. (A) The transcriptional start site of gyrB was mapped by primer extension analysis, using an oligonucleotide primer complementary to a sequence just upstream of the translational start site. Transcription initiates at two adjacent sites, indicated by stars, approximately 170 bp upstream of the predicted start of translation. (B) The gyrB promoter was compared with other known σ73 promoters, both cell-cycle regulated and constitutive; predominantly the former are shown [14-16,19,24-26]. The start sites and predicted -10 and -35 promoter elements are indicated above the sequence alignment; the published C. crescentus consensus sequence for genes transcribed by the common σ73 RNAP holoenzyme is shown below, where S stands for G or C, W indicates A or T, and N means any nucleotide [19,26]. The lines above the dnaX sequence indicate the previuosly identified RRF element .