Repression and inactivation of α-amylase in Thermomonospora species during growth on cellobiose.

Thermophilic actinomycetes establish themselves as numerically dominant bacterial populations in selected high temperature environments by virtue of their exoenzymic ability to degrade the complex polysaccharides in thermogenic plant biomass. When Thermomonospora curvata and Thermomonospora fusca were grown on a mixture of cellulose and starch in mineral salts minimal medium, α-amylase was repressed via inhibition of maltose uptake by cellobiose. Addition of cellobiose to exponential phase cells growing on maltose or maltotriose triggered rapid degradation of extant amylase in the culture fluid of wild-type cells, but not in a protease-deficient mutant of T. fusca. A serine protease purified from T. fusca caused inactivation of the amylase in culture fluid of the mutant when added at a concentration approximating to that of the wild-type strain. The chelating agent, EDTA, accelerated inactivation by the protease, while the presence of calcium or amylase reaction products protected the amylase. Therefore, during growth in an environment containing multiple polysaccharides, these thermophiles control the levels of their extracellular depolymerizing enzymes via both inducer exclusion and proteolytic inactivation.