Directional Cloning with an Insert That Contains EcoRI Sites
A student lab has a 4.0 kb plasmid with a single EcoRI site at position 100, BamHI at 800, HindIII at 1,500, and XhoI at 2,700 (positions in kb from a reference point). The gene of interest is a 1.2 kb insert that contains internal EcoRI and HindIII sites but no BamHI or XhoI sites. The student wants directional cloning into the plasmid multiple cloning site and will verify by restriction digestion and gel electrophoresis. The plasmid carries ampicillin resistance. A promoter lies immediately upstream of the BamHI site and transcribes clockwise toward XhoI. The lab has all four enzymes, T4 DNA ligase, and competent E. coli. The student proposes digesting both plasmid and insert with EcoRI alone, because it is the only enzyme that cuts the plasmid once and the insert once. The supervisor asks for a written justification of the enzyme choice and a predicted gel pattern.
1. Justify the choice of restriction enzymes for directional cloning, including why EcoRI alone is unsuitable and how the promoter orientation constrains the ligation.
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EcoRI is unsuitable for two reasons: it cuts inside the insert at least once, so digesting the insert with EcoRI would split the gene; and a single enzyme produces compatible ends at both ends of the plasmid, allowing self-ligation and insertion in either orientation. Directional cloning requires two different enzymes that (i) cut the plasmid once each in the multiple cloning site, (ii) do not cut the insert, and (iii) leave non-compatible ends. BamHI and XhoI meet these conditions: BamHI cuts the plasmid at 800 and XhoI at 2,700, both unique; neither cuts the 1.2 kb insert. Digestion excises the 1.9 kb stuffer and leaves a 2.1 kb backbone with BamHI and XhoI ends. The insert must be amplified or excised with BamHI at its 5' end and XhoI at its 3' end. Because the promoter lies immediately upstream of BamHI and transcribes clockwise toward XhoI, this orientation places the coding sequence downstream of the promoter. Ligation creates a 3.3 kb recombinant plasmid.
2. Predict the band sizes on an agarose gel for (a) the plasmid double digest, (b) the recombinant plasmid digested with EcoRI, and explain how the EcoRI pattern can reveal insert orientation.
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(a) Plasmid cut with BamHI + XhoI: the circle is cut at 800 and 2,700. The two fragments are 2,700 - 800 = 1.9 kb and 4.0 - 1.9 = 2.1 kb. (b) Recombinant: 4.0 - 1.9 + 1.2 = 3.3 kb. It has the backbone EcoRI site at 100 and one internal EcoRI site in the insert. In the correct orientation, the insert EcoRI site is 0.3 kb from the BamHI junction. The distance from EcoRI(100) clockwise to the junction at 800 is 0.7 kb; plus 0.3 kb into the insert = 1.0 kb. The other fragment is 3.3 - 1.0 = 2.3 kb. So EcoRI gives 1.0 and 2.3 kb bands (plus possibly linear 3.3 kb if digestion is incomplete). If the insert is reversed, its internal EcoRI site lies 1.2 - 0.3 = 0.9 kb from the BamHI junction, so the same digest gives 0.7 + 0.9 = 1.6 kb and 3.3 - 1.6 = 1.7 kb. Thus the band pattern distinguishes the two orientations; a 1.0/2.3 pattern indicates the desired orientation.
3. The plasmid has ampicillin resistance but no usable lacZ screen. Explain why blue-white screening would fail and propose an alternative verification strategy.
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Blue-white screening depends on a functional lacZ-alpha gene with the multiple cloning site inside it: insertion disrupts lacZ-alpha, so colonies cannot cleave X-gal and remain white, while empty plasmid colonies are blue. If this plasmid lacks lacZ-alpha or the multiple cloning site is not in frame within it, insertion does not change colony colour; all colonies would be blue or all white, so the screen gives no information. Ampicillin selects only for plasmid-bearing cells, not for recombinants, and self-ligated empty plasmid also survives. Alternative: colony PCR with one primer in the insert and one in the backbone; restriction digest of miniprep DNA with BamHI + XhoI (recombinant gives 2.1 + 1.2 kb, empty gives 2.1 + 1.9 kb); or Sanger sequencing across the junctions. If available, use a counterselection marker such as ccdB in the stuffer, or a promoterless reporter. Verify orientation by the EcoRI digest in the previous task.