While we think of plants as stationary, it doesn’t mean they’re passive. A plant needs to be able to react to its environment. Often signals are sent in the form of metabolites, small molecules that can be passed between cells. But how do cells react to metabolites, to change how a plant behaves? A new paper in Plant Physiology shows that cells can use ribosomes as sensors for many metabolites.
If DNA is the architectural plan of an organism, then ribosomes are the builders in cells that convert the plans into proteins. The DNA creates messenger RNA, usually written mRNA. The ribosome runs along the mRNA, reading it and creating proteins from amino acids as it works its way along. So it’s not just the DNA in a nucleus that matters. Plants also need the ribosome to express the genes from the DNA into proteins.
It’s not a simple matter of mRNA becoming the basis of a protein though. 20%-50% of mRNA contains a sequence called an upstream open reading frame, abbreviated to uORF. These uORFs usually stop an mRNA sequence from being translated. In plants, biologists have found a group of uORFs that plants share. They have been inherited from a common ancestor and have been conserved by their descendants, which suggests they do an important job that the plants need. As these uORFs are based around peptides, building blocks that the ribosome uses to build proteins, these uORFs are called Conserved peptide uORFs, or CPuORFs.
van der Horst and colleagues have been looking at these CPuORFs to see how they interact with the ribosome and found they don’t do it alone. This is where the metabolites come in. In the presence of some metabolites, like sucrose, polyamine or ascorbate, the CPuORF stalls the ribosome. As the ribosome stops, it never gets on to the main ORF (mORF) to start translating the mRNA into a protein.
This means that the ribosome is effectively the way a cell senses and reacts to its external environment. When metabolites are produced in or enter a cell they are in a position to bind to the ribosome and, combined with the CPuORFs, regulate gene translation. This changes the proteins the plant produces to react to its environment. This action shows a mechanism for metabolites to directly regulate gene expression, and the highly conserved nature of CPuORFs suggests this is a fundamental tool for plants.
Many different CPuORFs are present in plants and, possibly, each reacts to a specific metabolite, making the ribosome a metabolite multi-sensor.