The effects of temperature shifts on the synthesis of an extracellular proteinase and extracellular proteins in
Bacillus megaterium were studied. A shiftdown (42° to 28°C) brought about an immediate increase of proteinase synthesis by 70%. A shiftup (28° to 42°C) caused a temporary suppression of enzyme formation, which was, at least partially, owing to the inhibition of its mRNA synthesis. The shiftup also brought about a temporary decrease of excretion of all extracellular proteins.