A gene expression plasmid, pMALU7, for coding a fusion protein between protein A (SpA) and mutated firefly luciferase (Luc), was constructed. The fused gene was expressed in Escherichia coli and the resulting protein (SpA-LucTS) was purified with affinity chromatography. By changing a single amino acid, from Glu to Lys at the position 354 in the luciferase moiety, the thermostability of luciferase was improved.