Coordinate increase in activity of proteinase subunits of the 1300-kDa megaproteinase of Frankia strain BR: role of carbon source depletion and extracellular metabolites

We have recently described the presence of a high molecular mass multicatalytic proteinase complex (megaproteinase; 28 S, 1300 kDa) in Frankia strain BR. The complex dissociates into 11 low molecular mass proteinase subunits (40-19 kDa) when subjected to sodium dodecyl sulfate - gelatin - polyacrylamide gel electrophoresis. We show here that the activity of these proteinase subunits strongly increased after cessation of growth in stirred BAP-PCM mineral medium. Subsequent addition of either BAP medium components or sodium propionate alone, as carbon source, to a Frankia culture at the end of the exponential growth phase was found to prolong growth for 1 additional day, and to delay the increase in activity of the proteinase subunits for 3 days after cessation of growth. Addition of ammonium chloride alone, as nitrogen source, had no effect. On the other hand, when Frankia cells in the late exponential phase (3 days) were resuspended in a culture filtrate recovered from a 5-day-old culture and supplemented with BAP-PCM medium components, the biomass yield decreased to about 50%. Also, the activity of the proteinase subunits increased as soon as growth ceased. The ability of this culture filtrate to inhibit growth and stimulate the activity of proteinase subunits was partially lost by heating or was largely removed by DEAE-cellulose treatment. Thus, our findings indicate an extracellular control of Frankia megaproteinase activity, suggesting that carbon source depletion and probably accumulation of heat-sensitive growth-inhibiting metabolites in the medium are determining factors.