Improvement of the cloning linker of the bacterial expression vector pEX.

The pEX plasmids are a family of expression vectors for use in £. coB (1). Expression of cloned genes (as cro-lacZ fusion proteins) is controlled by the cI857 repressor gene. These vectors offer a number of dear advantages over other expression vectors; 1. Easy induction of gene expression (2 hours of Incubation at 42 *Q. Z A very high and consistent level of expression with virtually any open reading frame (up to L6 kb). 3. The extreme resistance of the expression product to proteorytic cleavage. 4. A convenient protocol for direct immunoscreening of colonies. For these reasons the vectors have been widely used eg. for the construction of cDNA libraries and for epitope mapping. However, cloning in these vectors is sometimes hampered by the limited number of unique restriction enzyme sites. We constructed pEXll-13 to increase the number of restriction endonudease sites present in the cloning linker (Figure 1). pEXll and pEX13 were made by Hgation of a synthetic ds-DNA molecule ( i n pEXl x Xma I x BamH I and pEX3, respectively.