A gene ‘stolen’ from bacteria gave us sight

do you know the book blind watchmaker Richard Dawkins? Maybe not as famous as his big hit The Selfish Genebut it’s a good read if you’re interested in the evolution of living beings.

In his first part, Dawkins is devoted to explaining the solution to one of the problems that most worried Charles Darwin: the evolutionary origin of something as complex as the eye from the gradual accumulation of small hereditary changes. On its pages, he explains how different animals solved this problem in different ways, from the squid’s organ of vision to the human’s organ of vision.

A recent paper published by a group led by Matthew Dougherty, a professor at the University of California, San Diego, explains the evolutionary origin of one of these “small changes.” And paradoxically, it could be defined as a “big robbery.”

Swing retinal and retinol

Over 500 million years ago, the ancestor of all vertebrates integrated the bacterial gene into its genome through a process of horizontal gene transfer. This gene evolved into the interphotoreceptor retinoid-binding protein (IRBP).

What is the IRBP protein used for? Our eyes can perceive light thanks to photoreceptor cells, which contain a light-sensitive molecule called cis-retinal. When a photon falls on it, it undergoes a series of changes that turn it into transretinal and then into retinol. The photoreceptor cell then sends a signal to the brain.

At this time, the retinol needs to be recycled. To do this, it leaves the photoreceptor cell and is packaged in IRBP, heading to the retinal pigment epithelium cell, where it becomes retinal again. From there, it is repackaged into IRBP and sent to the photoreceptor cell for reuse.

In short, IRBP is the “messenger” that delivers retinal or retinol to their respective destinations. Without IRBP there would be no refining and therefore no vision. In fact, some retinopathies are associated with decreased levels of this protein.

In the case of invertebrates, the process is quite different. The cis-retinal photosensitive molecule is converted to trans-retinal by blue light. And it’s recycled when orange light transforms trans-retinal back into cis-retinal. That is, photosensitive molecules should not leave the photoreceptor cell. The evolutionary path of the molecular mechanism of vision is completely different in invertebrates and vertebrates.

In search of bacterial origin

Peculiar similarities between the gene encoding human IRBP and the gene found in bacteria have been known since 2001, when the human genome was sequenced. However, it was assumed that this could be some kind of error in the analysis or contamination of the bacterial DNA sample.

Matthew Dougherty didn’t think so. His group has been looking for similar genes in the genomes of other vertebrate species that have not been found in other groups of animals. This may indicate a “leap” from bacteria to animals.

This is a case of the IRBP gene. It is present in all vertebrates, from the sea lamprey to the great apes. And the proteins most similar to it are bacterial peptidases, whose function is to process other proteins by proteolysis. Through this process, peptidase is able to break down a protein in order to reuse its amino acids to synthesize new proteins.

When the bacterial gene was passed on to a vertebrate ancestor, it underwent a series of modifications that resulted in the loss of its proteolytic activity in exchange for acquiring the function of binding retinal and retinol.

One of the transformations was a double gene duplication. Bacterial peptidase is a 295 amino acid protein with a single functional domain. What does it mean? Proteins work like tools, so let’s imagine that a bacterial peptidase is a single-edged razor. The double duplication resulted in 1247 amino acids and four domains. That is, he transformed into a Swiss army knife with four different tools.

So we have one more piece of the puzzle that explains the formation of the eye, but also brings up a few more mysteries. What was the process of this horizontal gene transfer between a bacterium and a vertebrate ancestor? Why did it only affect the cells involved in vision?

As often happens in science, when you answer one question, a hundred others appear.

Manuel Sanchez Angulo, Member of the Spanish Society for Microbiology. professor of microbiology, Miguel Hernandez University

This article was originally published on The Conversation. Read the original.