Conclusion

Other known DNA-binding domains are zinc-fingers, basic leucine zipper motifs and helix-turn-helix motifs, and each is considerably different to TAL repeats in structure and DNA-binding approach. RNA-binding proteins can be similar but differ in the segmented subunit TAL repeat.

Crystallographical analysis on dHax3 enables us to have a better understanding of how exactly each RVP loop are able to recognise specific DNA sequences. In summary, each TALE subunit is made up of two alpha helices connected by the RVP loop. The subunits are able to bind with each other to form a right-handed superhelix which is able to wrap around the DNA's major groove. The 12th and 13th residue on the RVP loop in each subunit is able interact independently with the DNA duplex without affecting their neighbour subunits. The code of DNA base recognition by the 12th and 13th amino acid residue has been deciphered and is key for all applicational uses.

Figure 6.1  The code of DNA base recognition by the 12th and 13th amino acid residue (Picture taken from Scholze & Boch, 2011)

However, scientists are still unable to explain how some of the repeat variable diresidues (RVDs) work in recognising specific bases such as how NI residues at position 12th and 13th are able to recognise Adenine and how NN recognises G/A base in the DNA duplex. 

The scientists from Tsinghua University (Deng et al, 2012) suggested that for NN, specific hydrogen bonding with  Asn¹³ may be preferential to G/A DNA bases at the end of their research paper. Further analysis of the TALE protein can be carried out using NMR to prove their hypothesis which will greatly facilitate rational design of the novel DNA-binding protein.