Isolation and characterization of ACC deaminase genes from two different plant growth-promoting rhizobacteria

We have recently proposed that one way that plant growth-promoting rhizobac teria (PGPR) stimulate plant growth is through the activity of the enzyme 1 -aminocyclopropane-1-carboxylate (ACC) deaminase, which causes a lowering o f plant ethylene levels resulting in longer roots. As part of an effort to understand the role of this enzyme in PGPR, the genes for ACC deaminase fro m two PGPR, Enterobacter cloacae CAL2 and UW4, have been isolated. These ge nes are highly homologous to the ACC deaminase genes from Pseudomonas strai ns 6G5 and F17 and similar to the ACC deaminase gene from Pseudomonas sp. s train ACP. The region downstream (i.e., at the 3'-terminal end) of the stra in UW4 ACC deaminase gene has a potential hairpin-like transcription termin ation site. The regions upstream of the strains UW4 and CAL2 ACC deaminase genes contain putative ribosome-binding sites; however, the promoter sequen ces have not yet been identified. Southern hybridization experiments suggest that there is a single copy of t he ACC deaminase gene in Enterobacter cloacae strains UW4 and CAL2 and that there may be several different types of ACC deaminase genes in different m icrobes. The cloned ACC deaminase gene can be expressed in Escherichia coli enabling this bacterium to grow on ACC as a sole source of nitrogen and co nfers upon both Escherichia coli and Pseudomonas spp. strains that are tran sformed with this gene the ability to promote the elongation of the roots o f canola seedlings.