A DNA fragment encoding the yeast GAL4 transcriptional activator DNA-b
inding domain (amino acids 1 93) was cloned into an Escherichia coli e
xpression vector such that the overproduced protein is tagged with six
histidine residues and a factor Xa protease cleavage site. The vector
also contains unique restriction sites at the 3' end of the gene to a
llow the construction of fusion proteins. These fusion proteins can ea
sily be purified to homogeneity and their activity tested in vitro.