Kinetics of the secretion of Bacillus subtilis levanase overproduced during the exponential phase of growth.

The Bacillus subtilis levanase structural gene sacC was expressed under the regulated control of sacR, the inducible levansucrase leader region, in a degU32(Hy) strain. In this genetic context, exocellular levanase is overproduced (0.5% of total protein) during the exponential phase of growth upon induction by sucrose at 37 degrees C and pH 7. No precursor form that comprised a signal peptide was detected in pulse-chase experiments. The subsequent release of the cell-associated processed protein is a slow event (t(1,2) = 80+/-10 s). The unfolding-folding transition of pure levanase monitored in vitro by the resistance to proteolysis was achieved within the same time range (t(1/2) = 50 s) under the same conditions of pH and temperature. Calcium ions, which modulate the rate and the yield of refolding, have a low affinity for the protein. Comparison of these results with those obtained previously with levansucrase and alpha-amylase overproduced in the same genetic and physiological context suggests that the precursor processing is more efficient in levanase and alpha-amylase than in levansucrase. This discrepancy could lie in information borne by the signal peptide sequence of these exoproteins. However, the rate of the ultimate stage of release of these three proteins, which includes the passage through the cell wall, is correlated with the rate of folding and appears to be independent of their molecular size.

Characterization of the rate-limiting step of the secretion of Bacillus subtilis alpha-amylase overproduced during the exponential phase of growth.

The Bacillus subtilis alpha-amylase gene, amyE, was expressed under the regulated control of sacR, the levansucrase leader region. The gene fusion including the complete amyE coding sequence with the signal peptide sequence was integrated into the chromosome of a degU32(Hy) strain deleted of the sacB DNA fragment. In this genetic contex, alpha-amylase is produced in the culture supernatant at a high level (2% of total protein) during the exponential phase of growth upon induction by sucrose. Pulse-chase experiments showed that the rate-limiting step (t1/2 = 120 s) of the secretion process is the release of a cell-associated precursor form whose signal peptide has been cleaved. The efficiency of this ultimate step of secretion decreased dramatically in the presence of a metal chelator (EDTA) or when the cells were converted to protoplasts. The hypothesis that this step is tightly coupled with the folding process of alpha-amylase occurring within the cell wall environment was substantiated by in vitro folding studies. The unfolding-folding transition, monitored by the resistance to proteolysis, was achieved within the same time range (t1/2 = 60 s) and required the presence of calcium. This metal requirement could possibly be satisfied in vivo by the integrity of the cell wall. The t1/2 of the alpha-amylase release step is double that of levansucrase, although their folding rates are similar. This perhaps indicates that the passage through the cell wall may depend on parietal properties (e.g. metal ion binding and porosity) and on certain intrinsic properties of the protein (molecular mass and folding properties).

Transcripts of the genes sacB, amyE, sacC and csn expressed in Bacillus subtilis under the control of the 5' untranslated sacR region display different stabilities that can be modulated.

When Bacillus subtilis levanase (SacC), alpha-amylase (AmyE) and chitosanase (Csn) structural genes were expressed under the regulated control of sacR, the inducible levansucrase (SacB) leader region in a degU32(Hy) mutant, it was observed that the production yields of the various extracellular proteins were quite different. This is mainly due to differences in the stabilities of their corresponding mRNAs which lead to discrepancies between the steady-state level of mRNA of sacB and csn on the one hand and amyE and sacC on the other. In contrast to levansucrase mRNA, the decay curves of alpha-amylase and levanase mRNAs obtained by Northern blotting analysis did not match the decay curves of their functional mRNA. This suggested that only a part of the population of the amyE and sacC transcripts was fully translated, while the others were possibly poorly bound to ribosomes and thus were only partially translated or not at all and consequently submitted to rapid endonuclease degradation. This hypothesis was substantiated by the finding that the introduction of a Shine-Dalgarno sequence upstream from the ribosome-binding site in the sacC transcript resulted in a fourfold increase in both the half-life of this transcript and the production of levanase. An additional cause of low-level levanase production is the premature release of mRNA by the polymerase. It was attempted to correlate this event with internal secondary structures of sacC mRNA.