Nucleotide chains, such as DNA and RNA, are synthesized by making copies from other chains. The copying process always happens in a “forward” direction, from one particular end to the other. During the process, the two chains of a double-stranded DNA that will be copied are separated and aligned in opposite directions to each other, complicating matters. “When DNA is replicated, one of the two chains can be copied, or synthesized, in a continuous manner while the other chain is synthesized in many fragments that need to be joined later,” says Min Yao from Hokkaido University. “One of the big questions in biology has been why cells don’t have a reverse-direction enzyme so that both chains can be synthesized efficiently.”
Recently, a group of enzymes was discovered, called Thg1-like proteins (TLPs), which were found to add nucleotides in the opposite direction. Examples of adding nucleotides in this direction are rare. TLPs are the exception and add nucleotides in the reverse direction to repair the “opposite end” of damaged RNAs. In a recently published study, Yao and her team used X-ray crystallography to uncover the structure of the TLP/RNA complex. This gave them insight into the complex mechanism that TLPs employ to add nucleotides in the reverse direction.