The unicellular green alga Chamydomonas reinhardtii, as distinct from higher plants synthesizing chlorophyll (Chl) only in the light, produces it also in darkness. The mechanisms of dark Chl biosynthesis were studied by biochemical and genetic methods on C. reinhardtii pigment LTS3 mutants, their revertants, and reverse mutants; LTS3 gene encodes a transcription factor regulating the genes of the key enzyme of Chl biosynthesis, magnesium chelatase (MgCh). The cells of LTS3 mutants accumulate in darkness the MgCh substrate, protoporphyrin IX, and produce orange colonies greening in the light. It turned out that the phenotype of these mutants is determined by a decrease of MgCh activity under conditions of heterotrophic growth. In the study of lts3 mutation suppression, two new nuclear genes of C. reinhardtii, SUP3 and SUP1, encoding factors regulating MgCh activity were found. SUP3 is linked with LTS3 and encodes a protein repressor of alternative to LTS3 pathway of MgCh regulation. Molecular identification of