PSEUDOMONAS-FLUORESCENS TN5-B20 MUTANT RA92 RESPONDS TO CARBON LIMITATION IN SOIL

Tn5-B20 (lacZ as reporter gene) transcriptional fusion mutants of Pseu domonas fluorescens R2f Rp(Gamma) were screened for their response to carbon limitation. Reporter gene expression was specifically induced b y this stress factor in one mutant, designated RA92, and to a lower ex tent by phosphorus and nitrogen starvation. Expression was not detecte d during exponential growth. Reporter gene activity in RA92 cells incr eased shortly after exposure to carbon limitation and reached a platea u level within 48 h. Survival of RA92 in mineral medium without carbon source, as determined by CFU enumerations, was impaired compared to t hat of the parent strain, R2f Rp(Gamma). Survival and induction of rep orter gene activity in exponentially grown as well as in carbon-starve d RA92 cells were measured after introduction into Ede loamy sand soil . Both the exponentially grown and carbon-starved RA92 cells responded to carbon limitation in Ede loamy sand soil by induction of reporter gene expression. RA92 cells introduced into soil developed enhanced re sistance to different stress factors in a fashion similar to that of t he parent strain R2f Rp(Gamma). RA92 CFU numbers dropped progressively in Ede loamy sand soil portions at a rate greater than that of its pa rent strain. The addition of glucose to Ede loamy sand inoculated with RA92 cells gave no reduction in the fitness of RA92. Also in soil mic rocosms planted with wheat, the fitness of RA92 was enhanced in the la yer 0-3 mm from the roots as compared to that in the deeper soil layer (3-8 mm from the roots).