Expression switching of three genes encoding light-independent protochlorophyllide oxidoreductase in Chlorella protothecoides.

Three chloroplast genes, chlL, chlN and chlB, encoding the light-independent protochlorophyllide oxidoreductase (LIPOR) in Chlorella protothecoides CS-41 growing either photoautotrophically, mixotrophically or heterotrophically, were all transcribed constitutively independent of illumination and presence of glucose. Steady-state amounts of all three transcripts in the light-grown cells were, however, approximately two- to three-fold greater than those in the dark-grown cells. In addition, Western blotting demonstrated that approximately the same amount of protein was present in cultures grown mixtrophically or heterotrophically both containing glucose. However, much less protein was in photoautotrophic cells. These results suggest that LIPOR activity depends on post-transcriptional and post-translational regulation. Moreover, the fact that LIPOR accumulates in the light-grown cells indicates that LIPOR, which was thought to work only in darkness, may partially account for the protochlorophyllide photoreduction in light. Therefore, LIPOR switches on both in light and in darkness.