Transcriptional control of a rRNA promoter of the nodulating symbiont Sinorhizobium meliloti.

We constructed a stable, low-copy-number plasmid containing a fusion between a Sinorhizobium meliloti rRNA promoter and gfp(mut3). When transformed into S. meliloti the resulting strain, Rm1021/pKW1, fluoresced in proportion to its growth rate during balanced growth. This strain also showed an unexpected behavior when grown to stationary phase in TY medium: the average cellular fluorescence increased through mid-exponential phase then decreased dramatically. The explanation for this appears to be that transcription from the rRNA promoter was shut off in mid-exponential phase and intracellular Gfp was diluted by continued cell growth.