Vectors expressible in a host that is the rhaBAD promoter region of the L-rhamnose operon operably linked to a transcriptional unit that is: a) a nucleic acid sequence which is heterologous to the host, and b) a prokaryotic signal sequence operably linked to the nucleic acid sequence. The prokaryotic signal sequence is selected from signal peptides of periplasmatic binding proteins for sugars, amino acids, vitamins and ions. The expression of the nucleic acid sequence is controlled by the promoter region. The vector is used for the regulated heterologous expression of a nucleic acid sequence in a prokaryotic host. This is an isolated and purified nucleic acid sequence expressible in a host is the promoter region of the L-rhamnose operon. There is a method for producing a polypeptide in a host using the vector.