Isolation of state transition mutants of Chlamydomonas reinhardtii by fluorescence video imaging

Oxygenic photosynthetic organisms adapt to varying light conditions by chan ging the distribution of light energy between Photosystem II (PS II) and ph otosystem I (PS I) during so-called state transitions. To identify the gene s involved in this process, we have exploited a simple chlorophyll fluoresc ence video-imaging technique to screen a library of nuclear mutants of Chla mydomonas reinhardtii for colonies grown on agar plates that are disturbed in their ability to regulate light energy distribution between PS I and PS II. Subsequent modulated fluorescence measurements at room temperature and 77 K fluorescence emission spectra confirmed that 5 mutants (0.025% of tota l number screened) were defective in state transitions. [P-32]orthophosphat e phosphorylation experiments in vivo revealed that in one of these mutants , designated stm1, the level of LHC II polypeptide phosphorylation was dras tically reduced compared with wild type. Despite WT levels of PS I and PS I I, stm1 grew photoautotrophically at reduced rates, compared with WT especi ally under low light conditions, which is consistent with an important phys iological role for state transitions. Our results highlight the feasibility of video imaging in tandem with mutagenesis as a means of identifying the genes involved in controlling state transitions in eukaryotic photosyntheti c organisms.