Phytase activity and its regulation in a rhizospheric strain of Serratia plymuthica.

Serratia plymuthica strain IC1270 isolated from the rhizosphere, possessing antagonistic activity towards a wide range of plant-pathogenic fungi, is able to hydrolyze phytate. Phytase activity was found intracellularly, while no activity was detected in the culture liquid. Optimum activity was found at pH 4-5; it completely disappeared at pH > 7.0 and 2.5. Phytase production was practically absent in the exponential phase and reached a maximum in the late stationary phase. Mutations of genes grrA and grrS, encoding GacA/GacS-like 2-component global regulatory system, or in gene rpoS encoding the sigma factor RpoS subunit of RNA polymerase, led to a deficiency in phytase production. Introduction into mutants of the respective wild-type genes cloned into the wide-range plasmid pJFF224-NX under the control of the bacteriophage T4 gene 32 promoter complemented this deficiency. This is the first report implicating the GacA/GacS global regulators and RpoS factor in phytase production in bacteria.