[PCR-based assembly of the DNA sequence coding for tetanus toxin C fragment].

Objective: To develop a PCR-based method for gene assembly of tetanus toxin C fragment (TETC) DNA sequence from a large number of oligodeoxyribonucleotides (oligos).

Methods: To allow for its cloning and expression in Lactococcus lactis, the TETC gene sequence was designed according to the known TETC gene sequence (GenBank accession number M12739, 367-1719) and the amino acid coding in Lactococcus lactis. The sequence contained 1383 nucleotides (nt) with Sal I site added to its 5' end and Xho I and Hind III sites to its 3' end. There were 209 synonymous codon substitutions in the designed gene sequence as compared with the sequence reported in GenBank for amino acid coding in Lactococcus lactis and elimination of the restriction site of EcoR I and Kpn I. The 1380 nt of the sequence was divided into 68 oligos designated as TETC 1 to TETC 68, each containing 40 nt. A 16 nt oligos designated as TETC 69 was designed as the downstream primer. The TETC 1-24 fragment was acquired using the oligos TETC 1 to TETC 24 by PCR-based gene assembly method, and the TETC 23-46 and TETC45-68 fragments were assembled similarly. The full-length TETC gene was assembled using TETC 1 and TETC 69 as the primers when the 3 fragments were mixed. The target gene was gel-purified and digested with Sal I and Hind III, followed by ligation to the pBluescript II SK(+) and digestion with the same enzymes. The positive clones were confirmed by restriction enzyme excision and sequencing.

Results: Three 500-bp fragments were acquired by PCR-based gene assembly, and the full-length TETC gene was obtained from the 3 fragment mixed at a equal concentration by a second PCR-based gene assembly using TETC 1 and TETC 69 as the primers. The target gene was cloned to pBluescript II SK(+) vector, and sequence analysis of the positive clones indicated that the assembled sequence was identical to the designed coding sequence of TETC gene.

Conclusion: PCR-based assembly of the synthesized constitutive gene fragments into the complete sequence can be an effective strategy for synthesis of long DNA sequences in vitro.