Temporal expression patterns of the gyrB promoter fusions during the C. crescentus cell cycle. Transcription of the gyrB-lacZ fusions contained on the indicated plasmids was examined during synchronized growth by immunoprecipitation of pulse-labeled β-galactosidase, as described in Material and Methods. The lanes, each revealing the β-galactosidase synthesized during one five-minute interval, are aligned beneath a diagram showing the corresponding stage in the cell cycle, as determined by light microscopy. 'Cell division units' refer to fractions of one complete cell cycle, the duration of which is approximately 140 minutes under the experimental conditions employed here. Samples were labeled every 30 minutes or every 20 minutes (pJEZP5).