Characterization of G-Quadruplexes in Chlamydomonas reinhardtii and the Effects of Polyamine and Magnesium Cations on Structure and Stability.

Chlamydomonas reinhardtii is a green alga with a very GC-rich genome (67%) and a high density of potential G-quadruplex-forming sequences (PQSs). Using the Ensembl Plants DNA database, 19 PQSs were selected, and their ability to fold in vitro was examined using four experimental methods. Our results support in vitro folding of 18 of the 19 PQSs selected for study. The high physiological polyamine concentrations in C. reinhardtii create unique conditions for studying G4 folding. We investigated whether high polyamine concentrations affect the stability and structural fold of two polymorphic G4s selected from the cohort of PQSs. The two polymorphic G4s selected were found to be greatly stabilized when studied at the physiologically high polyamine concentrations. Lastly, the effects of physiologically relevant Mg2+ concentrations were tested on both of the polymorphic G4s, and one of the G4s shifted from a dynamic mixture of folds to favor a parallel fold in the presence of Mg2+. Our work supports the concept of folding of G4s under the unique conditions observed in C. reinhardtii, and these structures, being located in promoter regions of DNA repair and photosynthetic genes, might be relevant structures in the physiology of C. reinhardtii.