22Trans-methylation reactions are intrinsic to cellular metabolism in all living organisms. In land 23 plants, a range of substrate-specific methyltransferases catalyze the methylation of DNA, RNA, 24 proteins, cell wall components and numerous species-specific metabolites, thereby providing 25 means for growth and acclimation in various terrestrial habitats. Trans-methylation reactions 26 consume vast amounts of S-adenosyl-L-methionine (SAM) as a methyl donor in several 27 cellular compartments. The inhibitory reaction by-product, S-adenosyl-L-homocysteine 28 (SAH), is continuously removed by SAH hydrolase (SAHH) activity, and in doing so 29 essentially maintains trans-methylation reactions in all living cells. Here we report on the 30 evolutionary conservation and multilevel post-translational control of SAHH in land plants. 31 We find that SAHH forms oligomeric protein complexes in phylogenetically divergent land 32 plants, and provide evidence that the predominant enzyme is a tetramer. By analyzing light-33 stress-induced adjustments occurring on SAHH in Arabidopsis thaliana and Physcomitrella 34 patens, we demonstrate that both angiosperms and bryophytes undergo regulatory adjustments 35 in the levels of protein complex formation and post-translational modification of this 36 metabolically central enzyme. Collectively, these data suggest that plant adaptation to 37 terrestrial environments involved evolution of regulatory mechanisms that adjust the trans-38 methylation machinery in response to environmental cues. 39 40 41 3 42 INTRODUCTION 43 Land plants have evolved sophisticated biochemical machineries that support cell metabolism, 44 growth and acclimation in various terrestrial habitats. One of the most common biochemical 45 modification occurring on biological molecules is methylation, which is typical for DNA, 46 RNA, proteins, and a vast range of metabolites. Trans-methylation reactions are therefore 47 important in a relevant number of metabolic and regulatory interactions, which determine 48 physiological processes during the entire life cycle of plants. Trans-methylation reactions are 49 carried out by methyl transferases (MTs), which can be classified into O-MTs, N-MTs, C-MTs 50 and S-MTs based on the atom that hosts the methyl moiety [1,2]. All these enzymes require S-51 adenosyl-L-methionine (SAM) as a methyl donor [3]. Among MTs, O-MTs form a large group 52 of substrate-specific enzymes capable of methylating RNA, proteins, pectin, monolignols as 53 well as various small molecules in various cellular compartments [2]. 54 55 The availability of SAM is a prerequisite for methylation, while the methylation reaction by-56 product, S-adenosyl-L-homocysteine (SAH) is a potent inhibitor of MT activity and must 57 therefore be efficiently removed [4]. To ensure the maintenance of SAM-dependent trans-58 methylation capacity, SAH is rapidly hydrolysed by S-adenosyl-L-homocysteine hydrolase 59 (SAHH, EC 3.3.1.1) in a reaction that yields L-homocysteine (HCY) and adenosine (ADO) 60 [5]. Subsequent...