Plasmid-to-Plasmid Southern Blot Analysis Validates the Presence of Nucleotide Binding Site ('nbs') Sequences in Cloned Plasmids

Southern blot analysis is an important molecular biology technique for identifying a specific sequence in DNA samples. Although it is no longer used extensively in recent years, the steps and underlying principles of Southern blot are applicable to modern biology. High sensitivity and limited background are keys to successful Southern blots, whereas obtaining good quality and quantity of genomic DNA as starting materials and detecting a single/low copy target sequence in the genome can be challenging. To ensure student success in performing the technique for the first time, a modified "plasmid-to-plasmid" Southern blot was implemented to confirm the presence of grape nucleotide-binding site ("nbs") sequences in cloned plasmids like those described previously. The plasmid DNA and a control plasmid, pSCA7 (T1-T3-W6) containing a known grape "nbs" sequence, were digested with restriction enzymes, followed by agarose gel electrophoresis. The DNA band corresponding to the "nbs" sequence of the pSCA7 (T1-T3-W6) was extracted from the gel for PCR digoxigenin (DIG) probe synthesis. At the same time, the cloned plasmid DNA and its digested DNA fragments were blotted from the gel onto nylon membranes to be hybridized with the DIG probe followed by the detection for "nbs" sequences. Students successfully performed Southern blots to confirm the presence of "nbs" sequences in their cloned plasmids and wrote up the results following the format of scientific research papers. They learned the principles and applications of Southern blot and gained hands-on experience with associated techniques.