Abstract: Novel fusion reporter genes, fusion reporter proteins, and an improved reporter system for measuring the relative activity of a promoter sequence. A luxAB fusion gene of the present invention is particularly useful as a reporter gene and is derived from the fusion of a luxA gene and a luxB gene from Vibrio harveyi. The gene products of the luxA and luxB genes are the .alpha.- and .beta.-subunits, respectively, of a bacterial luciferase. A fusion protein encoded by a luxAB fusion gene is a single active protein and is particularly useful as a reporter protein having luciferase activity. An advantage of such a reporter system to assay gene expression in many cells which contain FMNH.sub.2, such as bacterial and yeast cells, is that an immediate and quantitative assessment of gene expression may be made from real-time light measurements using intact cells.