PARENT SESSION
Posters P6B Photosynthetic acclimation: Mechanisms and gene expression. Abstracts (531-578)

Isolation and characterization of mutants in Photosystem I degradation. Jianying Zhang^*,1, Galina Gulis¹, Walter Evans¹, Nathan Henderson¹, Kevin Redding¹, ¹ Depts. of Chemistry and Biological Sciences; University of Alabama, 120 Lloyd Hall, 6th Ave, Tuscaloosa, AL, U.S

ABSTRACT- To study the process of photosystem I degradation, we are using both biochemical and genetic approaches to identify the components involved. Using an in vitro assay, we found that degradation of PSI appeared to be a multi-event process, with disassembly of the complex preceding proteolysis of the subunits. Addition of soluble proteases caused rapid loss of immuno-detectable PSI polypeptides and cleavage of the major PSI polypeptides in interhelical loops without accelerating disassembly of the complex. The in vitro degradation process was time- and temperature-dependent and required divalent cations (Zn²⁺ > Co²⁺, Ca²⁺ > Mn²⁺, Ni²⁺, Cu²⁺ > Fe²⁺, Mg²⁺), but did not require ATP, GTP, or soluble chloroplast proteins. We have isolated mutants that suppress the nonphotosynthetic phenotype of a point mutation in PsaA (PsaA-H408Q) that accelerated PS1 degradation. Photosynthetic growth tests, fluorescence induction and immunoblot were used to confirm that the suppressor mutants possess increased steady-state levels of the mutant PSI. Use of " chloramphenicol chases " demonstrated that some of the suppressor mutants retard degradation of the mutant PS1 in vivo. Moreover, thylakoid mutants from these suppressor mutants displayed defects in degradation of the mutant PSI in vitro. All of the suppressor mutations behave as Mendelian loci. To study the specificity of suppression effect, we crossed one representative suppressor mutant to mt⁺ strains harboring mutations in other chloroplast genes. We found this suppression effect was not specific to PSI; it could also suppress point mutations in PSII. We have crossed this suppressor mutant to the interfertile and highly polymorphic S1-D2 strain and are in the process of mapping the mutation in order to identify the affected gene by positional cloning.

KEY WORDS: degradation, photosystem I, suppressor, mutant