Light stress-regulated two-helix proteins in Arabidopsis thaliana related to the chlorophyll a/b-binding gene family

The chlorophyll a/b, chlorophyll a/c, and chlorophyll a/a light-harvesting proteins are part of an extended gene family that also includes the transie ntly expressed stress proteins, the flips (early light-induced proteins). F our flip homologue proteins, encoded by single-copy nuclear genes, have bee n identified in the Arabidopsis thaliana database. These proteins were divi ded into two groups according to the expression pattern under light-stress conditions and the predicted secondary structure. Group one included two me mbers of the Flip family with three predicted transmembrane helices and a g ene expression strictly related to light stress. Group two included two pro teins, the Seps (stress-enhanced proteins), which possessed two predicted t ransmembrane segments. The transcripts of Sep1 and Sep2 were present under low light conditions, but their level increased 4- to 10-fold during illumi nation of plants with high-intensity light. Preliminary data indicated that the induced transcripts were translated in vivo. Other physiological stres s conditions, such as cold, heat, desiccation, salt wounding, or oxidative stress, did not significantly influence the expression of Sep genes. In vit ro import of radioactively labeled precursors of Seps into isolated chlorop lasts confirmed the thylakoid membrane localization of these proteins. Cons idering the predicted protein structure and homology to other pigment-anten na proteins, the two-helix Seps might represent an evolutionary missing lin k between the one- and three-helix antenna proteins present in pro- and euk aryota.