A simple and efficient method is described for the isolation of extension fragments of known DNA sequences by polymerase chain reaction (PCR) using a single specific primer. With this method, size-selected genomic DNA fragments are ligated to a plasmid Vector (pGEM-4Z) which contains sequencing primers and the population of chimeric plasmids is used for transforming Escherichia coli. DNA is extracted from an aliquot of the resulting mini-library and PCR performed using a sequence-specific primer and either of the standard sequencing primers of the plasmid vector. This method appears to be more Versatile than inverse PCR (IPCR), since: ii) the DNA sequence needed as the specific primer can be as short as about 20 nucleotides (nt); (ii) the DNA templates to be used in PCR are available in high amount, thus facilitating all manipulations; and (iii) if relinearization of the DNA by restriction enzyme digestion is desired before the PCR reaction, many restriction sites can be chosen from the vector polylinker. Using this method, we have isolated the genomic 5' region of the carrot bifunctional dihydrofolate reductase-thymidylate synthase-encoding gene dhfr-ts using a 21-nt sequence of the 5' region of the dhfr-ts cDNA clone as the specific primer.
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