The present invention relates to a method of producing a recombinant protein comprising using a selection method other than antibiotics. In particular, it relates to a stable host/vector system based on the pyrC gene complementation designed to produce high level of heterologous recombinant protein in Escherichia coli. The expression system of the present invention allows rapid selection of plasmid containing cells during the cloning phases and lead to high protein expression during fermentation. This system has a strong selective efficiency, especially during the induction phase, leading to the selection of an almost homogeneous and stable plasmid bearing cell population. Moreover the productivity of the culture is to a large extent better than the one based on antibiotic resistance. This elevated vector stability combined with its high productivity fulfils the requirements for heterologous protein production in Escherichia coli to an industrial level.