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A series of transposons are described which contain the gusA gene, encoding -glucuronidase (GUS), expressed from a variety of promoters, both regulated and constitutive. The regulated promoters include the tac promoter which can be induced by IPTG, and nifH promoters which are symbiotically activated in legume nodules. One transposon contains gusA with a strong Shine-Dalgarno translation initiation context, but no promoter, and thus acts as a promoter-probe transposon. In addition, a gus operon deletion strain of Escherichia coli, and a transposon designed for use in chromosomal mapping using PFGE, are described. The GUS transposons are constructed in a mini-Tn5 system which can be transferred to Gram-negative bacteria by conjugation, and will form stable genomic insertions. Due to the absence of GUS activity in plants and many bacteria of economic importance, these transposons constitute …