Researchers have made a groundbreaking discovery regarding the IRBP gene, which is responsible for encoding a light-sensitive pigment carrier protein used in the eyes of all vertebrates. Interestingly, this gene is absent in invertebrates, but a closely related version of IRBP has been found in bacteria. It appears that half a billion years ago, the common ancestor of vertebrates acquired this DNA from bacteria, shedding light on the evolutionary origins of this key gene in vertebrate vision.
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| The former bacterial gene functions in all vertebrate eyes / Tom Woodward |
While prokaryotes, such as bacteria, are known to readily exchange DNA fragments through horizontal gene transfer, a mechanism distinct from vertical transmission from parents to offspring, this process is less common in animals, particularly in complex organisms like vertebrates. In fact, through complete sequencing of the human DNA, scientists have identified only around 200 genes that could potentially be traced back to bacteria. However, subsequent studies have revealed that the majority of these genes have origins that are exclusive to animals, rather than being borrowed from bacteria.
Recently, biologists have confirmed that one of these genes, IRBP, indeed has a bacterial origin. Through DNA sequence analysis of hundreds of species, it has been found that IRBP first appears in the ancestors of vertebrates and is not present in other animals, indicating that it was acquired through borrowing. Further research has identified a distant relative of IRBP in bacteria, specifically among the genes that encode certain proteins of the peptidase family. These findings have been published in an article authored by Matthew Daugherty and colleagues, which is available in the journal PNAS.
The IRBP gene is responsible for encoding retinol-binding protein 3 (RBP3), which functions in the space between the light-sensitive cells of the eye and its pigment epithelium. When the pigment molecule captures a photon and undergoes a structural change, triggering a signal, RBP3 facilitates the transfer of the pigment to the epithelium, where it is restored to its original shape before being returned to the receptor cells. This process plays a crucial role in vision in all vertebrates but is absent in invertebrates.
Based on the findings obtained by Daugherty and his colleagues, it is evident that the common ancestor of all vertebrates likely acquired the corresponding gene from a bacterium, which then underwent changes and adaptations in its functions. In bacteria, this gene codes for a protein involved in the breakdown and utilization of other proteins. However, in animals, this gene lost its original activity, underwent duplication events resulting in four copies, and underwent multiple mutations, eventually evolving into a mechanism for transporting pigment molecules, as seen in the case of IRBP.