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TAL EFFECTORS- Transcriptor activator-like effectors.

How is the amino acid sequence of each module (common and specific aa)?

These proteins secreted by bacteria can recognise specific DNA sequences due to their

structure and composition. TAL effectors are made up of repeated stretches of amino

acids. The most distinctive characteristic of TAL effectors is a central repeat domain

containing between 1.5 and 33.5 repeats that are usually 34 residues in length. It was

observed that the C-terminal repeat was generally shorter and so it is usually called

“half repeat”.

These repeated stretches differ from each other at only two amino acids at position 12and 13 which were called RVDs (repeat variable residues). These are the two

determining factors that control whether that stretch of the TAL protein will bind to an

A, C, T, or G in the genome, giving the TAL effectors the ability to target various areas

of the genome.

A typical repeat sequence is LTPEQVVAIASHDGGKQALETVQRLLPVLCQAHG

although amino acids in position 12 and 13 will change deepening on the sequence of 

DNA bases in the TAL effector’s target site.

Figure 1 1

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What is the structure of individual modules?

The structure of each of the repeat modules is very precise. The 33-35, normally 34 aa

modules form two double α-helixes. As we can see in   figure 2, the first α-helix also

known as smaller or a α-helix is form by aa 3 to 11. Residues 15 to 33 form the larger-

also known as b α-helix. Residues 12 and 13 are located in the loop joining both helixes

1 Page 717. Science Vol 335. February 2012. www.sciencemag.org. 

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together with a fixed Gly. As we can see in the figure this structure is curled due to van

der Waals iterations between the helixes. Residue located at position 12 is the one in

charged of stabilizing and maintaining this structure and allowing residue 13 to

recognise DNA bases. To do so, the α-helixes point away from DNA bases when they

meet the DNA.

Figure 1 22 

What is the structure of a tandem of at least 11.5 subunits?

Previous individual module structure forming the

α- helixes will conform a right handed super-

helix structure when joining DNA so DNA

double strand can fit inside this super-helix. As

we can see in   figure 2, each 11 modules form a

turn with the RVD in contact with the DNA so

they can target it. Studies showed that when the

super helix is not bound to DNA it has a pitch of 

60 Å. However if the TAL bounds DNA the pitch

will lower to 35 Å, nearly DNA major groove

pitch where the TAL joins.

In this super helical structure the small or a α-

helixes will form the internal layer whether big or

 b α- helixes will compose a more external layer.

2 Page 717. Science Vol 335. February 2012. www.sciencemag.org. 3 Solving the Structure of a Protein That Shows Promise as a DNA-Targeting Molecule for

Gene Correction, Therapy. 2012. http://www.newswise.com/articles/  

Figure 1 3