TALEN
/ TAL-en /
TALENs were the second generation of programmable DNA scissors, and they fixed the most annoying flaw of zinc-finger nucleases: the targeting was now easy to design. They borrow their DNA-reading part from proteins called TAL effectors, made by Xanthomonas bacteria that inject them into plants to switch on plant genes. The remarkable thing about these proteins is how simply they read DNA.
A TAL effector is built from a chain of nearly identical repeat units, and each repeat reads exactly one DNA base. Which base a repeat reads is set by just two amino acids inside it — a simple, almost alphabet-like code: one pair of amino acids means 'recognise A', another means 'recognise C', and so on. To target any sequence you simply line up the matching repeats in order, like spelling a word letter by letter. A TALEN bolts this designed repeat array onto the same FokI cutting domain used by zinc-finger nucleases, and again works in pairs flanking the target so the two FokI halves meet and make a double-strand break.
Because the one-repeat-reads-one-base code is so clean, TALENs were far more reliable and predictable to build than zinc-finger nucleases, and they could be aimed at almost any sequence. They are still valued today for high specificity and for targeting places CRISPR struggles with. Their drawback is bulk: each TALEN is a long string of repeats, awkward to assemble and to deliver into cells. When CRISPR arrived — reprogrammable by simply ordering a short RNA rather than building a whole new protein — it largely eclipsed TALENs for everyday use, though TALENs remain a strong choice in many applications.
To target the sequence 5'-TGACCT-3', you assemble a TALEN with repeats reading T, then G, then A, then C, then C, then T in order — one repeat per letter, like spelling the word out.
One repeat per base makes targeting as simple as spelling.
TALENs are very specific, but they are still proteins built fresh for each target. CRISPR did not win by being more precise — it won by being reprogrammable through a cheap, quickly synthesised guide RNA instead of a re-engineered protein.