BETA-GLUCURONIDASE (GUS) TRANSPOSONS FOR ECOLOGICAL AND GENETIC-STUDIES OF RHIZOBIA AND OTHER GRAM-NEGATIVE BACTERIA

A series of transposons are described which contain the gusA gene, enc oding beta-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 ar e symbiotically activated in legume nodules. One transposon contains g usA with a strong Shine-Dalgarno translation initiation context, but n o promoter, and thus acts as a promoter-probe transposon. In addition, a gus operon deletion strain of Escherichia coli, and a transposon de signed for use in chromosomal mapping using PFGE, are described. The G US transposons are constructed in a mini-Tn5 system which can be trans ferred to Gram-negative bacteria by conjugation, and will form stable genomic insertions. Due to the absence of GUS activity in plants and m any bacteria of economic importance, these transposons constitute powe rful new tools for studying the ecology and population biology of bact eria 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 an d spatial localization of marked bacteria to be carried out.