The invention relates to polynucleotides and recombinant cell strains comprising the polynucleotides and the uses thereof for the delivery of the polypeptides encoded by the polynucleotides to a subject in need thereof. In particular, the invention refers to polynucleotides comprising a polypeptide of interest, auxotrophy-complementing genes and the use thereof in a mycobacterial double auxotrophic host cell to achieve the stable expression of the polypeptide of interest by using an antibiotic-free plasmid selection system.