Vectors for directional cloning

1. A method of inducing expression of a DNA sequence of interest in a host cell, comprising contacting a recombinant host cell which is deficient in rhamnose catabolism, and has a recombinant DNA molecule comprising a rhamnose-inducible promoter operably linked to an open reading frame for a heterologous RNA polymerase, with rhamnose and an expression vector comprising a promoter for the heterologous RNA polymerase operably linked to a DNA sequence of interest, so as to induce expression of the DNA sequence of interest, wherein the host cell is an E. coli cell.

2. The method of claim 1 , wherein the DNA sequence of interest is flanked by two restriction enzyme sites, wherein one of the flanking restriction enzyme sites is for a first restriction enzyme which has infrequent restriction sites in cDNAs or open reading frames from at least one species and generates single-strand DNA overhangs, and wherein another flanking restriction enzyme site is for a second restriction enzyme which has infrequent restriction sites in cDNAs or open reading frames from at least one species and generates ends that are not complementary to the overhangs generated by the first restriction enzyme.

3. The method of claim 1 , wherein the expression vector further comprises a transcription terminator sequence.

4. The method of claim 3 , wherein the transcription terminator sequence is that of rrnB.

5. The method of claim 1 , wherein the expression vector is a plasmid.

6. The method of claim 5 , wherein the plasmid further comprises a selectable marker gene.

7. The method of claim 5 , wherein the plasmid further comprises a sequence which specifies a high copy number.

8. The method of any of claim 5 , wherein the plasmid further comprises a sequence which reduces vector multimerization.

9. The method of claim 1 , wherein the host cell does not express one or more rhamnose catalytic genes.

10. The method of claim 1 , wherein the rhamnose-inducible promoter comprises a rhaBAD promoter.

11. The method of claim 1 , wherein the RNA polymerase is a phage RNA polymerase.

12. The method of claim 2 , wherein the expression vector further comprises a transcription terminator sequence that is rrnB.

13. The method of claim 2 , wherein the expression vector is a plasmid comprising a selectable marker gene and a sequence which specifies a high copy number.

14. The method of claim 13 , wherein the plasmid further comprises a sequence which reduces vector multimerization.

15. The method of claim 2 , wherein the rhamnose-inducible promoter comprises a rhaBAD promoter.

16. The method of claim 2 , wherein the RNA polymerase is a phage RNA polymerase.

17. The method of claim 1 , wherein the recombinant host cell is rhaBAD − .