No, we are not cyanobacteria – but where is my photolyase?

By Dr. Hannah Sivak | March 12, 2022

I am happy to be human. It can be tough sometimes but it is also fun. Conversely, being a cyanobacterium may be a bit of a drag, at least when compared with the life of a human: they are born, they photosynthesize, they divide, they die. No love, no music, no movies. But cyanobacteria have an advantage I envy, and I want to tell you about it.

We like to lie on a chair and take in the sun, by the sea. It feels good. Unfortunately, our skin likes UV (because it makes us happy) but our skin DNA doesn’t. UV damages skin DNA in many bad ways and the results may include hyperpigmentation, wrinkles, and, sometimes, skin cancer. Fortunately, our skin (and the rest of our body) has some tools to repair damaged DNA. This is very important for survival because DNA can be damaged in so many ways: oxidants, the sun, “accidents”.

DNA can be fixed in several ways. However, humans and other mammals are missing one of the repair mechanisms that bacteria have: the enzyme photolyase. We lost it during evolution!

Why can’t we make vitamin C? During evolution we lost an enzyme needed to make it. The transformation of L-gulonolactone into ascorbic acid is catalyzed by the enzyme L-gulono-gamma-lactone oxidase in those organisms that have it. We don’t have it so we need to ingest (and apply to our skin) vitamin C for collagen to be finished the way it should. Just like that, we lost photolyase on the way to becoming humans. What are photolyases? They are light-dependent DNA repair enzymes, they are activated by blue light and repair UV-induced DNA damage by removing pyrimidine dimers. A perfect enzyme.

All placental mammals lack the ability to harness light for DNA repair after UV damage. DNA photolyases are ancient proteins, they use the energy from light to revert UV-induced DNA damage. This is a highly efficient process, as lesions induced by the UV component of sunlight are repaired by visible light from the same source. Lacking these enzymes means relying on less efficient ones that cut out the damaged nucleotides rather than repairing them directly. Why would living creatures lose such a useful trait? There is a hypothesis, called the “nocturnal bottleneck”. This “Jurassic Park” theory goes like this: when dinosaurs and other large reptiles ruled the land, early mammals were forced to hide during daylight, becoming nocturnal, and just lost an enzyme that they didn’t need anymore. When the big monsters were gone and we went back to enjoy the sun, our photolyase was gone.

Here is a great gimmick to sell skincare products: tell your potential clients that you can give them back, topically, the photolyase they lost. Never mind that our DNA is located inside the nucleus, which is located inside the live cells. A topically applied photolyase, assuming it’s still active, would have to go through the stratum corneum and then go through cell membranes and nuclear membrane, barriers on which our life depends.

So, no. I don’t believe that topical bacterial photolyases can repair our skin DNA. It shouldn’t hurt us either, because a bacterial extract (Micrococcus lysate) will be dead on arrival anyway (and so will the enzymes in it). But please don’t spend money on products that promise you to replace what evolution took from us. Just be careful when you enjoy the sun and wear sunscreen.

References

Yang H. Conserved or lost: molecular evolution of the key gene GULO in vertebrate vitamin C biosynthesis. Biochem Genet. 2013 Jun;51(5-6):413-25. doi: 10.1007/s10528-013-9574-0. Epub 2013 Feb 12. PMID: 23404229.

Rohner, N. (2018). Evolution: A Dark Past. Current Biology, 28(20), R1190–R1192. doi:10.1016/j.cub.2018.08.045