Abstract: The present invention relates to expression of recombinant proteins by use of a bacterial host expression vector which expresses a recombinant protein under the control of a first regulatory expression element, and expresses a selectable marker under the control of a second regulatory expression element, which second element is mutated such that expression of the selectable marker is at reduced levels relative to that directed by such an unmutated expression element. Such an expression vector in a suitable bacterial host (a) allows ease of purification of the recombinant protein of interest ("the recombinant protein") since less selectable marker is present to interfere with the purification of the recombinant protein, and (b) increases the amount of recombinant protein that is produced by the bacterial host cell.

Abstract: Improved expression vectors for amplifying expression of DNA sequence encoding a desired polypeptide beyond that expected by the vector copy number. The vector contains a DNA sequence within the replicon of the vector encoding RNA I and the primer RNA for initiation of DNA replication and their regulatory regions, said DNA sequence having an AT to GC mutuation at position 3029 in the two strands of pBR322; a promoter of RNA I or primer RNA and a restriction site for insertion of a DNA sequence encoding a desired polypeptide into the vector and operatively linking the DNA sequence to the expression control sequence. The AT to GC mutation causes an increase in the vector copy number and further amplifies the expression of the desired polypeptide operatively linked to the RNA I or primer RNA promoter beyond that expected by the copy number increase.