In vivo genetic engineering: homologous recombination as a tool for plasmid construction

Cell-associated degradation affects the yield of secreted engineered and heterologous proteins in the Bacillus subtilis expression system

A series of chimeric alpha-amylase genes derived from amyL, which encodes the liquefying alpha-amylase from Bacillus licheniformis, were constructed in vitro using gene splicing techniques. The gene constructs were cloned in Bacillus subtilis, where their ability to direct the synthesis and secretion of active alpha-amylase was determined. Detectable alpha-amylase activity was observed for some, but not all, of the chimeric proteins. Studies on the secretion of wild-type AmyL and its chimeric derivatives revealed that, whilst these proteins were stable in the extracellular milieu, all were subject to some degree of degradation during secretion. The chimeric enzymes were degraded to a greater extent than the native enzyme. These findings suggest that cell-associated proteolysis is a significant problem affecting the use of B. subtilis as host bacterium for the production of heterologous proteins.

Chaperone-mediated activation in vivo of a Pseudomonas cepacia lipase

An extracellular Pseudomonas cepacia lipase, LipA, is inactive when expressed in the absence of the product of the limA gene. Evidence has been presented that LimA is a molecular chaperone. The lipA and limA genes have been cloned in separate and independently inducible expression systems in Escherichia coli. These systems were used to test the molecular chaperone hypothesis by investigating whether LimA could activate presynthesized prelipase and whether presynthesized LimA could activate newly synthesized prelipase. The results show that LimA cannot activate presynthesized prelipase and that presynthesized LimA can activate only a limited number of de novo synthesized prelipase molecules. Co-immunoprecipitation of prelipase/lipase with LimA generated a 1:1 complex of prelipase/lipase and LimA. The results suggest that a 1:1 complex of LipA and LimA is required for prelipase processing and secretion of active lipase.

celA from Bacillus lautus PL236 encodes a novel cellulose-binding endo-beta-1,4-glucanase

celA from the cellulolytic bacterium Bacillus lautus PL236 encodes EG-A, an endo-beta-1,4-glucanase. An open reading frame of 2,100 bp preceded by a ribosome-binding site encodes a protein with a molecular mass of 76,863 Da with a typical signal sequence. The NH2-terminal active domain of EG-A is not homologous to any reported cellulase or xylanase and may represent a new family of such enzymes. A 150-amino-acid COOH-terminal peptide is homologous to noncatalytic domains in several other cellulases (A. Meinke, N.R. Gilkes, D.G. Kilburn, R.C. Miller, Jr., and R.A.J. Warren, J. Bacteriol. 173:7126-7135, 1991). Upstream of celA, a partial open reading frame encodes a 145-amino-acid peptide which also belongs to the family mentioned. Zymogram analysis of extracts from Escherichia coli and supernatants of Bacillus subtilis and B. megaterium, including protease-deficient mutants thereof, which express celA, revealed two active proteins, EG-A-L and EG-A-S, with Mrs of 74,000 and 57,000, respectively. The proportion of EG-A-L to EG-A-S depends on the extracellular proteolytic activity of the host organism, indicating that EG-A-S arises from posttranslational proteolytic modification of EG-A-L. Since EG-A-S has an NH2 terminus corresponding to the predicted NH2-terminal sequence of EG-A, processing appears to take place between the catalytic and noncatalytic domains described. EG-A-L and EG-A-S were purified to homogeneity and shown to have almost identical characteristics with respect to activity against soluble substrates and pH and temperature dependency. EG-A-L binds strongly to cellulose, in contrast to EG-A-S, and has higher activity against insoluble substrates than the latter. We conclude that the COOH-terminal 17,000-Mr peptide of EG-A-L constitutes a cellulose-binding domain.

Cloning and expression of an amylase gene from Bacillus stearothermophilus

In the industrial process of liquefying starch to make glucose or high fructose syrups, it is crucial that the amylase used is stable and active at about 105 degrees C at pH 6.5 or preferentially at a lower pH. The amylase from Bacillus licheniformis is well suited for this purpose but it is possible that other amylases might perform even better. Therefore, we cloned and characterized amyS encoding a heat-stable alpha-amylase from Bacillus stearothermophilus. Using a newly developed method for creating exact gene fusions by in vivo recombination, we attempted to increase expression of amyS in Bacillus subtilis. However, only by introducing the amyS gene into B. licheniformis, we obtained significantly better yields.