Proteolytic activity under nitrogen or sulfur limitation

Sand cultures were used to evaluate the effect of C, N, and S ratio on protein degradation by soil microorganisms. Sand was inoculated with soil and amended with defined nutrient media to produce limitation for C, N, or S. Limitation for N or S resulted in reduced biomass (total protein) and increased proteolytic activity as indicated by measurements of dye released from a commercial protease substrate (azocoll). Carbon limitation had little effect on proteolytic activity. As expected, utilization of carbon (glucose) was dependent upon the availability of N or S. Protein synthesis inhibitors (chloramphenicol and cycloheximide) suppressed proteolytic activity, suggesting a need for new gene expression in the response of organisms to N or S stress. Correlations of proteolytic activity and biomass among treatments revealed distinctly different relationships depending upon the availability of C, N, or S. The results of this experirnent support a role of proteolytic activity in response of microorganisms to N or S deprivation and suggest that protease activity in soil is more strongly influenced by regulatory signals than by standing biomass.