Abstract: Methods and compositions are provided for regulated expression of polypeptides in transformed bacterial hosts. A novel class of plasmid cloning vehicles includes a DNA sequence coding for the desired polypeptide (on an insertion site therefor) linked for transcriptional expression in reading phase with four functional fragments derived from the lipoprotein gene of E. coli. The plasmids further include a DNA sequence coding for a specific segment of the E. coli lac promoter-operator, which is positioned in the proper orientation for transcriptional expression of the desired polypeptide, as well as a separate functional E. coli lacI gene coding for the associated repressor molecule which can interact with the lac promoter-operator to prevent transcription therefrom. Expression of the desired polypeptide is under the control of both the constitutive promoter and the inducible promoter, although transcription from either promotor is normally blocked by the repressor molecule.