DNA macroarray and real-time PCR analysis of two nuclear photosystem I mutants from Chlamydomonas reinhardtii reveal downregulation of Lhcb genes but different regulation of Lhca genes

In photoautotrophic organisms, the expression of nuclear genes encoding plastid proteins is known to be regulated at various levels. In this study, we present the analysis of two non-photosynthetic mutants (CC1051 and TR72) from the unicellular green alga Chlamydomonas reinhardtii. Both mutant strains show a defect in the processing of chloroplast psaA mRNA, and therefore they are assumed to be defective in photosystem I (PSI) assembly. We have performed macroarray experiments with trans-splicing mutants CC1051 and TR72 in order to analyse putative pleiotropic effects of nuclear-located mutations leading to a non-functional PSI. To the best of our knowledge, this is the first example of Chlamydomonas cDNA macroarray analysis comparing the transcriptional regulation of nuclear genes in wild-type and photosystem I mutants. The macroarray results demonstrated a transcriptional downregulation of members of the Lhcb gene family more than 2-fold in both mutant strains. In addition, real-time RT-PCR experiments found a 4- to 16-fold reduction in transcript levels of several Lhca genes in TR72; whereas in CC1051, no significant change in transcript levels was observed. Taken together, our data suggest that a signal is transmitted from the chloroplast to the nucleus that serves to regulate the level of light harvesting polypeptides in the organelle.