Differential dependence of levansucrase and alpha-amylase secretion on SecA (Div) during the exponential phase of growth of Bacillus subtilis

SecA, the translocation ATPase of the preprotein translocase, accounts for 0.25% of the total protein in a degU32(Hy) Bacillus subtilis strain in loga rithmic phase. The SecA level remained constant irrespective of the demand for exoprotein production but dropped about 12-fold during the late station ary phase. Modulation of the level of functional SecA during the exponentia l phase of growth affected differently the secretion of levansucrase and al pha-amylase overexpressed under the control of the sacB leader region, The level of SecA was reduced in the presence of sodium azide and in the div341 thermosensitive mutant at nonpermissive temperatures. Overproduction of Se cA was obtained with a multicopy plasmid bearing secA, The gradual decrease of the SecA level reduced the yield of secreted levansucrase with a concom itant accumulation of unprocessed precursor in the cells, while an increase in the SecA level resulted in an elevation of the production of exocellula r levansucrase, In contrast, alpha-amylase secretion was almost unaffected by high concentrations of sodium azide or by very low levels of SecA, Secre tion defects were apparent only under conditions of strong SecA deprivation of the cell. These data demonstrate that the alpha-amylase and levansucras e precursors markedly differ in their dependency on SecA for secretion. It is suggested that these precursors differ in their binding affinities for S ecA.