Summary: A series of transposons are described which contain the gene, encoding β-glucuronidase (GUS), expressed from a variety of promoters, both regulated and constitutive. The regulated promoters include the promoter which can be induced by IPTG, and promoters which are symbiotically activated in legume nodules. One transposon contains with a strong Shine-Dalgarno translation initiation context, but no promoter, and thus acts as a promoter-probe transposon. In addition, a operon deletion strain of , 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 powerful new tools for studying the ecology and population biology of bacteria in the environment and in association with plants, as well as for studies of the fundamental molecular basis of such interactions. The variety of assays available for GUS enable both quantitative assays and spatial localization of marked bacteria to be carried out.


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