Evolution of a New Protein-Coding Gene
How does a new protein-coding gene evolve? In most cases, new genes are essentially modified copies of older ones. An old gene produces a protein with a particular function; after one of many well known types of random events creates an extra copy of this gene, one copy may, through mutation, produce a protein with a different function. But how do protein coding genes arise in the first place? How do you get from a non-coding sequence to a coding one? A research group from the Chinese Academy of Sciences discovered a completely new protein coding gene in yeast - one that appears to have arisen spontaneously from a non-protein-coding sequence. The gene in question, BSC4, is only found in the brewer's yeast, Saccharomyces cerevisiae, not in the genomes of any of the other 81 fungal species the researchers scanned. BSC4, although it's function is unknown, does have some sort of functional role, since it has turned up in a variety of genetic surveys for function. What we have is a functional protein-coding gene in S. cerevisiae and not in any other yeast species. So where did this gene come from? There must be some similar stretch of DNA somewhere in other yeast species. And in fact, there is: in other yeast species, closely related to S. cerevisiae, there is a stretch of non-protein-coding DNA fairly similar the DNA of BSC4. (38% similar - which for non-coding DNA is reasonably good similarity.) In the other species, the corresponding stretch of DNA cannot produce protein - the DNA does not code for an unbroken stretch of amino acids. But in S. cerevisiae, this DNA does code for protein. It appears that through mutation (and selection), a non-coding region was converted into a coding region. But there is still another problem: in order for the BSC4 gene to actually generate protein, the DNA has to be transcribed into RNA, which becomes the template for the protein. You can have mutations produce a protein-coding gene in DNA, but unless that DNA is actually transcribed, your protein-coding gene is useless. As it turns out, in the other yeast species, the ones that don't have an actual BSC4 gene, the corresponding stretch of DNA is transcribed - suggesting that in these other species, there might be a non-coding RNA gene in this there. What appears to have happened is this: in some ancestral yeast species, living millions of years ago, there was a functional, but non-protein-coding RNA gene being transcribed and doing something (we don't know what yet) in the cell. In the line leading to S. cerevisiae, mutations converted this non-coding gene into a protein-coding one - BSC4. Does BSC4 play a role similar to that of it's non-coding predecessor? We have no idea. The functional mystery still needs to be unraveled, but it is clear that BSC4 is a fascinating example of how new protein coding genes can be generated.