Using enhanced green fluorescent protein (EGFP) promoter fusions to study gene regulation at single cell and population levels.

Reporter gene fusions based on the enhanced green fluorescent protein (EGFP) are powerful experimental tools that allow real-time changes in gene expression to be monitored both in single cells and in populations. Here we describe the development of a chromosomally integrated transcriptional reporter fusion in Listeria monocytogenes that allows real-time measurements of gene expression. To construct a single copy of an EGFP-based fluorescent reporter fused to a promoter of interest (Px) in L. monocytogenes, a suicide shuttle vector carrying the Px::egfp gene fusion is first constructed in Escherichia coli (as an intermediate host). Then, the vector is transformed into L. monocytogenes and integrated into its chromosome by homologous recombination within the selected promoter region. Subsequently, analysis of fluorescence exhibited by cells carrying a single copy reporter can be performed under selected experimental conditions by stringent sample preparation, optimized image acquisition, and processing of the digital data with the image analysis freeware ImageJ. Thus, the methodology described here can be adapted to investigate the activity and regulation of any promoter in L. monocytogenes both at the cell and population levels.