Isolation and characterization of Chlamydomonas suppressor mutants in Photosystem 1 degradation

A suppressor genetics approach is being used to identify the components involved in the process of Photosystem 1 (PS1) degradation. By UV mutagenesis treatment, suppressor mutants were isolated from a strain expressing PsaA/PsaB double-mutant PS1 that is normally degraded quickly. They restore the ability of photosynthetic growth by increasing the steady-state level of the mutant PS1 complex. These suppressors were found not to revert to the original point mutations but to develop nuclear mutations. The in vitro degradation assays on thylakoid membranes from the suppressor mutants demonstrated that two categories of suppressor were generated---those that increased expression and those that decreased degradation. The representative suppressor mutant 3U-1 in the latter class was chosen for further studies. After confirming that the mutation retarded degradation of the mutant PS1 by both in vivo and in vitro degradation assays, the map-based cloning was performed on the suppressor mutant to identify the gene. A putative DegP protease that shows a similarity to DegP 10 protease from Arabidopsis was proposed as a candidate involved in the degradation of PS1; other potential players are discussed as well.