In <i>Pseudomonas syringae</i> pv. phaseolicola, Expression of the <i>argK</i> Gene, Encoding the Phaseolotoxin-Resistant Ornithine Carbamoyltransferase, Is Regulated Indirectly by Temperature and Directly by a Precursor Resembling Carbamoylphosphate

Abstract

Pseudomonas syringae pv. phaseolicola synthesizes a non-host-specific toxin, phaseolotoxin, and also synthesizes a phaseolotoxin-resistant ornithine carbamoyltransferase (ROCT) to protect itself from its own toxin. ROCT is encoded by argK, which is expressed coordinately with phaseolotoxin synthesis at 18 degrees C. To investigate the regulatory mechanisms of this system, null mutants were constructed for argK, argF (encoding the phaseolotoxin-sensitive OCTase [SOCT]), and amtA (encoding an amidinotransferase involved in phaseolotoxin synthesis). The argF mutant did not exhibit arginine auxotrophy when grown in M9 medium at 28 degrees C, because under this condition SOCT was replaced by ROCT. This loss of thermoregulation of argK was apparently caused by accumulation of carbamoylphosphate, one of the substrates of SOCT. Carbamoylphosphate, which has a structure similar to that of the inorganic moiety of phaseolotoxin, was used in induction assays with wild-type P. syringae pv. phaseolicola and was shown to be able to induce argK expression in M9 medium at 28 degrees C. These results indicate that argK expression is independent of temperature and is regulated directly by a compound resembling the inorganic moiety of phaseolotoxin.