One of the most successful fluorescent proteins, used as a reporter of gene expression in many bacterial, plant and animals, is green fluorescent protein and its modified forms, which also function well in cyanobacteria. However, these fluorescent proteins do not allow rapid and economical quantitation of the reporter gene product, as does the popular reporter gene lacZ, encoding the enzyme β-galactosidase. We provide here a protocol for the in situ localization of β-galactosidase activity in cyanobacterial cells. This allows the same strain to be used for both a simple, quantitative, colorimetric assay with the substrate ortho-nitrophenyl-β-galactoside (ONPG) and for sensitive, fluorescence-based, cell-type localization of gene expression using 5-dodecanolyaminofluorescein di-β-D-galactopyranoside (C12-FDG).
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