A method for producing a heterologous protein comprising:
culturing a coryneform bacterium having a genetic construct for secretory expression of the heterologous protein; and
collecting the heterologous protein produced by secretory production,
wherein the coryneform bacterium has been modified so that the number of molecules of a RegX3 protein per cell is reduced as compared with a non-modified strain,
wherein the genetic construct comprises, in the direction from 5' to 3', a promoter sequence that functions in the coryneform bacterium, a nucleic acid sequence encoding a signal peptide that functions in the coryneform bacterium, and a nucleic acid sequence encoding the heterologous protein, and
wherein the heterologous protein is expressed as a fusion protein with the signal peptide.
2. The method according to claim 1, wherein the RegX3 protein is a protein
defined in (a), (b), or (c) mentioned below:
(a) a protein comprising the amino acid sequence of SEQ ID NO: 65;
(b) a protein comprising the amino acid sequence of SEQ ID NO: 65, but which includes substitution, deletion, insertion, and/or addition of 1 to 10 amino acid residues, wherein said protein has a function as a response regulator of a SenX3-RegX3 system;
(c) a protein comprising an amino acid sequence showing an identity of 90% or higher to the amino acid sequence of SEQ ID NO: 65, wherein said protein has a function as a response regulator of a SenX3-RegX3 system.

3. The method according to claim 1 or 2, wherein the number of molecules of the RegX3 protein per cell is reduced by reducing the expression of a regX3 gene, or by disrupting a regX3 gene.
4. The method according to any one of claims 1 to 3, wherein the number of molecules of the RegX3 protein per cell is reduced by deleting a regX3 gene.

5. The method according to any one of claims 1 to 4, wherein the coryneform bacterium has been further modified so as to harbor a phoS gene encoding a mutant PhoS protein,
6. The method according to claim 5, wherein the mutation is a mutation of replacing an amino acid residue corresponding to the tryptophan residue at position 302 in SEQ ID NO: 4 with an amino acid residue other than aromatic amino acid and histidine residues in a wild-type PhoS protein.
7. The method according to claim 6, wherein the amino acid residue other than aromatic amino acid and histidine residues is a lysine residue, alanine residue, valine residue, serine residue, cysteine residue, methionine residue, aspartic acid residue, or asparagine residue.
8. The method according to claim 6 or 7, wherein the wild-type PhoS protein is a protein defined in (a), (b), or (c) mentioned below:

(a) a protein comprising the amino acid sequence of SEQ ID NO: 4, 26, 27, 28, 29, or 30;
(b) a protein comprising the amino acid sequence of SEQ ID NO: 4, 26, 27, 28, 29, or 30, but which includes substitution, deletion, insertion, or addition of 1 to 10 amino acid residues, wherein said protein has a function as a sensor kinase of a PhoRS system;
(c) a protein comprising an amino acid sequence showing an identity of 90% or higher to the amino acid sequence of SEQ ID NO: 4, 26, 27, 28, 29, or 30, wherein said protein has a function as a sensor kinase of a PhoRS system.

9. The method according to any one of claims 1 to 8, wherein the signal peptide is a Tat-dependent signal peptide.
10. The method according to claim 9, wherein the Tat-dependent signal peptide is a signal peptide selected from the group consisting of a TorA signal peptide, SufI signal peptide, PhoD signal peptide, LipA signal peptide, and IMD signal peptide.
11. The method according to claim 9 or 10, wherein the coryneform bacterium has

been further modified so that the expression of one or more genes selected from genes encoding a Tat secretion system is increased as compared with a non-modified strain.
12. The method according to claim 11, wherein the genes encoding a Tat secretion system consists of a tatA gene, tatB gene, tatC gene, and tatE gene.
13. The method according to any one of claims 1 to 8, wherein the signal peptide is a Sec-dependent signal peptide.
14. The method according to claim 13, wherein the Sec-dependent signal peptide is a signal peptide selected from the group consisting of a PS 1 signal peptide, PS2 signal peptide, and SlpA signal peptide.
15. The method according to any one of claims 1 to 14, wherein the genetic construct further comprises a nucleic acid sequence encoding an amino acid sequence comprising Gln-Glu-Thr between the nucleic acid sequence encoding the signal peptide that functions in the coryneform bacterium and the nucleic acid sequence encoding the heterologous protein.
16. The method according to claim 15, wherein the genetic construct further comprises a nucleic acid sequence encoding an amino acid sequence used for enzymatic digestion between the nucleic acid sequence encoding the amino acid sequence comprising Gln-Glu-Thr and the nucleic acid sequence encoding the heterologous protein.
17. The method according to any one of claims 1 to 16, wherein the coryneform bacterium is a bacterium belonging to the genus Corynebacterium.
18. The method according to claim 17, wherein the coryneform bacterium is Corynebacterium glutamicum.
19. The method according to claim 18, wherein the coryneform bacterium is a modified strain derived from Corynebacterium glutamicum AJ12036 (FERM BP-734)

or a modified strain derived from Corynebacterium glutamicum ATCC 13869.
20. The method according to any one of claims 1 to 19, wherein the coryneform
bacterium is a coryneform bacterium in which the number of molecules of a cell surface layer protein per cell is reduced.