The present invention is based, in part, on our discovery of a way to configure expression cassettes so that the expression of a selectable marker protein, which is critical for the growth or survival of a cell, also results in the expression of a protein of interest in a biological cell. Accordingly, in one aspect, the invention features a genetically modified cell (e.g., a bacterial cell) that includes a chromosomally integrated or cytoplasmic expression cassette that includes a first nucleic acid sequence encoding a protein of interest and a second nucleic acid sequence encoding a selectable marker protein. The regulatory sequence (e.g., the sequence encoding a functional promoter) that drives expression of the required selectable marker protein also drives expression of the protein of interest. For that reason, we may refer to their expression as being “linked” or “functionally couple.”
