Monitoring promoter activity in a single bacterial cell by using green and red fluorescent proteins

We investigated the possibility of monitoring promoter activity with flow cytometry by using green fluorescent protein (GFPmut2) and red fluorescent protein (drFP583) in a single bacterial cell. The drFP583 was used as an intrinsic marker of the bacterial cells, because it was expressed constantly in Escherichia coli MC1061 strain. The GFPmut2 expressed under the control of the Hg(2+) ion inducible mer promoter/operator, was used to study promoter activity. Over 75% of the cells were positive for red and green fluorescence in flow cytometric analysis. The average green fluorescence of the whole population increased from 6.7 to 1700 when the mercury concentration was increased from 0 to 1 x 10(-4) M, while the red fluorescence was unaffected by the mercury concentration. These results show that gfpmut2 and drFP583 could be expressed under different promoters in one bacterial cell and measured independently with a flow cytometer.