Vectors for directional cloning

1. A vector encoding a fusion polypeptide prepared by ligating

a DNA fragment comprising a first opening reading frame, a 5′ end generated after cleavage of a first recognition site with a first restriction enzyme which generates an end compatible with an end generated after cleavage of a SgfI recognition site by SgfI, and a 3′ end generated after cleavage of a second recognition site for a second restriction enzyme which generates a blunt end, and

a DNA segment comprising a second open reading frame that begins with an ATG but does not end with an in-frame stop codon, a 3′ end generated after cleavage of a third recognition site with a third restriction enzyme which generates an end compatible with an end generated after cleavage of a SgfI recognition site by SgfI, and a 5′ end generated after cleavage of a PmeI site with PmeI,

wherein the first restriction enzyme is SgfI, the third restriction enzyme is SgfI, or both the first and third restriction enzymes are SgfI, wherein ligation of the blunt end and the 5′ end generated after cleavage of the PmeI site provides an in-frame TAA stop codon for the first open reading frame, and wherein ligation of the SgfI compatible ends yields a third open reading frame comprising the first and second open reading frames which encodes the fusion polypeptide.

2. The vector of claim 1 wherein the DNA segment further comprises a promoter 5′ to the ATG of the second open reading frame.

3. The vector of claim 1 wherein the first and third recognition sites are cleaved by SgfI and yield an exchange site comprising GCGATCGCnATGG (SEQ ID NO: 92), wherein n is C, A, T or G.

4. The vector of claim 3 wherein n is C.

5. The vector of claim 1 wherein the first and third recognition sites are cleaved by SgfI and yield an exchange site comprising GCGATCGCnATG (SEQ ID NO: 93), wherein n is C, A, T or G.

6. The vector of claim 1 wherein the first and third recognition sites are cleaved by SgfI and yield an exchange site comprising GCGATCGCnATG (SEQ ID NO: 93), wherein n is C.

7. The vector of claim 1 , wherein the first open reading frame does not begin with ATG.