Cellulose is a main component of the cell walls of plants, and is a typical biomass which is effectively used for, for example, making clothing, paper, and building material. Hitherto, in order to effectively use this biomass, attempts have been made to convert cellulose into sugars and further into energy-producing substances by use of a cellulose-degrading enzyme. Since alkaline cellulase derived from alkalophilic bacteria belonging to the genus Bacillus had been found by Horikoshi et al., (see, for example, Patent Document 1 and Non-Patent Document 1), application of cellulase to a heavy-duty detergent for clothing, which had been considered difficult, has become possible, and alkaline cellulase produced by alkalophilic bacteria belonging to the genus Bacillus (see, for example, Patent Documents 2 to 4) has been incorporated into detergents for clothing.
In recent years, as genetic engineering has been progressed, a variety of useful substances have been produced by use of microorganisms on an industrial scale, and enzymes for detergents have been mass-produced through genetic recombination techniques. In such industrial-scale production of useful substances by use of microorganisms, improvement of productivity thereof is one major topic of interest. In order to achieve such productivity improvement, attempts have been made to grow useful-substance-producing bacteria through mutagenesis or a similar genetic technique, and to develop host microorganisms suitable for genetic recombination.
In Gram-positive bacteria (including bacteria belonging to the genus Bacillus (e.g., Bacillus subtilis)) and Gram-negative bacteria such as Escherichia coli, intracellularly synthesized protein (immature secretory protein) is generally transported extracellularly via the so-called Sec pathway, which is a type of transport system. In Bacillus subtilis, the Sec pathway functions by, for example, SecA, which serves as a motor for extracellular release of secretory protein; three Sec proteins (SecY, SecE, and SecG), which constitute a main portion of a transport channel through which the secretory protein passes; and SecDF, which is a cofactor of the transport channel.
There have been reported an expression vector capable of overexpressing a secG gene, which encodes SecG protein (Patent Document 5), and a Gram-positive bacterium in which expression of a secG gene is altered through modification of the ribosome-binding site of the secG gene (Patent Document 6). As has been shown, for example, growth of Bacillus subtilis whose secG gene is disrupted is inhibited during production of a heterologous protein. As has also been reported, mutation of a secY gene in Escherichia coli inhibits low-temperature growth thereof or protein secretion (Non-Patent Document 2).
However, there have not yet been reported data indicating that overexpression of a secG gene or a secY gene in living cells promotes secretion or production of a target protein. In addition, it has not yet been fully elucidated how overexpression of the other genes involved in the Sec pathway affects secretion or production of a target protein.    Patent Document 1: JP-B-1975-28515    Patent Document 2: JP-B-1985-23158    Patent Document 3: JP-B-1994-030578    Patent Document 4: U.S. Pat. No. 4,945,053    Patent Document 5: JP-A-2001-510046    Patent Document 6: US Patent Application No. 2003/0157642    Non-Patent Document 1: Horikoshi & Akiba, Alkalophilic Microorganisms, Springer, Berlin (1982)    Non-Patent Document 2: Cell, 32: 789 (1983)