particular promoters can be placed upstream of the "luc" gene, that is, the "luc" sequence can be fused to the promoter sequence at dna level. if such construct is not too large in size, it can simply be introduced into eukaryotic cells using plasmids. this approach is widely used to study the activity of a given promoter in a given cell/tissue type, since the amount of light produced by the luciferase is directly proportional to the promoter activity . in addition to studying the promoters, firefly luciferase assays offer the option of studying transcriptional activators: in these experiments typically the gal4/uas_system is used and its gal4 upstream activating dna sequence (uas) is placed upstream the "luc" gene while the different activators or the different variants/fragments of the same activator are fused to the gal4 dna binding module at protein level. this way the transcriptional activity of the different gal4 fusion proteins can be directly compared using light as a readout. .