Chloroplast Protein Acetyltransferases – Novel Players in the Regulation of Photosynthesis

Ivanauskaite Aiste

Life on Earth as we know it depends on the availability of oxygen and organic carbon compounds, such as sugars. The source of these basic ingredients of life is photosynthesis, light energy from the Sun is used to convert water and carbon dioxide into oxygen and sugars. In plants, photosynthesis takes place in the chloroplast, a subcellular organelle that hosts the light harvesting machinery and the protein complexes responsible for photosynthesis.

The ability to harvest light energy also makes the photosynthetic machinery vulnerable to damage caused by too much light. In my thesis I have focused on the regulatory mechanisms that plants have evolved to avoid unnecessary damage, while making sure that enough light energy is available for photosynthesis. Many of these regulatory mechanisms depend on small chemical modifications of the photosynthetic machinery and result in the rapid adjustment of the light harvesting components. While many of these modifications have been studied for decades and their function is well-known, the development of research methods has allowed the discovery of new modifications in the chloroplast. One such modification is acetylation, the addition of an acetyl group to a protein, which was recently found to affect many components of the photosynthetic machinery.

The aim of my thesis was to find out the role(s) of protein acetylation in photosynthesis by studying a group of acetyltransferase enzymes that are responsible for the acetylation reactions in the chloroplast. This study has shown that the GNAT2 acetyltransferase as well as two newly discovered acetylation targets of GNAT2 are required for the adjustment of light harvesting in plants. In addition, several GNAT acetyltransferases affect plant metabolism, especially the amount of lipid-compounds, ascorbic acid (commonly known as vitamin C) and amino acids. These results deepen our understanding of how plants adjust to the constantly changing light conditions in nature.