Tau-beta-galactosidase, an axon-targeted fusion protein.

The most commonly used enzymatic reporter molecule, Escherichia coli beta-galactosidase (beta-gal; beta-D-galactoside galactohydrolase, EC 3.2.1.23), fails to readily diffuse into axons; consequently, the morphologies of beta-gal-labeled neurons cannot directly be determined. For analysis of neuronal pathfinding and synaptic connectivity, this information is essential. We have constructed an axon-targeted beta-gal reporter by fusing the cDNA encoding the bovine microtubule-binding protein, tau, to lacZ, the E. coli gene encoding beta-gal. This reporter labels cell bodies and axons when expressed by developing and adult Drosophila neurons. It also reveals the entire cellular extent of nonneuronal cells such as muscle fibers and glia. To generate neuronal markers for studies of Drosophila neural development, we constructed a tau-beta-gal enhancer-trap transposon. From 1500 independent lines generated by mobilization of this transposon, we have isolated a set of useful markers for specific subsets of neurons, glia, and muscles. Since the tau cDNA-lacZ reporter utilizes bovine tau, it may also effectively target beta-gal in vertebrate neurons and prove to be a useful reagent for the analysis of vertebrate nervous systems.