Post-translational modifications (PTMs) of proteins such as phosphorylation have been shown to play pivotal roles in the regulation of photosynthesis. However, the study of these small modifications has long been hindered by methodological limitations. In recent years, advances in mass spectrometry methods have enabled the identification of a myriad of PTMs affecting proteins in all subcellular compartments. Especially interesting is the high prevalence of protein acetylation in the chloroplast, and more specifically in photosynthetic proteins.

The acetylation machinery in the chloroplast consists of seven acetyltransferase enzymes that belong to the General control non-repressible 5-related N-acetyltransferase (GNAT) superfamily. The chloroplast-localized GNATs (GNAT1-5, GNAT7 and GNAT10) have been shown to catalyse two types of protein acetylation reactions: the addition of an acetyl group to the free N-terminus and the acetylation of an internal lysine residue of a protein. In addition, GNAT1 and GNAT2 function as metabolite acetyltransferases in the biosynthesis of melatonin. The presence of such a versatile group of plastid-localized GNAT acetyltransferases points to the importance of this PTM in the chloroplast.

In this thesis, I have focused on elucidating the physiological role(s) of a group of the newly identified chloroplast GNAT acetyltransferases with a special focus on photosynthesis. My work has shown that GNAT2 is required for the regulation of excitation energy distribution between the photosystems through state transitions. Specifically, the formation of the PSI-LHCII complex is hindered in the gnat2 mutant, although no defects were detected in LHCII phosphorylation, which was previously considered to be the main determinant of state transitions. Additionally, GNAT2 was shown to be essential for the dynamic responses of the thylakoid membrane to changes in light conditions. GNAT1, GNAT2, GNAT4, GNAT7 and GNAT10 have a marked effect on the metabolome of Arabidopsis thaliana, especially on the accumulation of oxylipins, lipids and two acetylated amino acids. Finally, I have characterized two previously unknown thylakoid membrane proteins acetylated by GNAT2 and revealed their involvement in the dynamic adjustment of the light harvesting antenna of photosystem II.