Inducible expression system

A composition comprising a host cell and at least one inducer;

• wherein the host cell comprises at least one extrachromosomal expression construct comprising at least one first Escherichia coli sugar-inducible promoter and at least one polynucleotide sequence encoding a polypeptide, wherein at least one said polynucleotide sequence is to be transcribed from said first inducible promoter, wherein the polypeptide lacks a signal peptide;
• and
• wherein the host cell comprises at least one extrachromosomal expression construct comprising at least one second inducible promoter, selected from the group consisting of an Escherichia coli sugar-inducible promoter and a propionate-inducible promoter, that is inducible by at least one inducer that is different than that of said first inducible promoter;
• and
• wherein none of said inducible promoters is a lactose-inducible promoter;
• and
• wherein the host cell has an altered gene function of at least one gene that affects the reduction/oxidation environment of the host cell cytoplasm;
• and
• wherein the host cell has a reduced level of gene function of at least one gene encoding a protein that metabolizes an inducer of at least one said inducible promoter;
• and
• wherein each inducer is selected from the group consisting of a sugar and propionate.