Genetic Interplay between Chloroplast Protein Import and Thylakoid Complex Assembly in Arabidopsis thaliana

The aim of the present thesis is to further our knowledge of chloroplast protein import regulation by identifying potential novel regulatory components using the model species Arabidopsis thaliana. A forward genetic screen with the pale chloroplast protein import mutant tic40 uncovered two second-site suppressors (stic1 and stic2). The stic1 muta-tions map to the known thylakoid biogenesis factor ALB4, which suggest a novel genet-ic interaction between chloroplast protein import and thylakoid biogenesis. The genetic interaction between TIC40 and ALB4 is shown to be specific, and the chloroplast pro-tein import defect of tic40 mutants can be suppressed by stic1/alb4 mutants. Further-more, the ALB4 and STIC2 proteins are shown to interact in a common pathway, the abrogation of which leads to deteriorated thylakoid ultrastructure, suggesting that both ALB4 and STIC2 are involved in thylakoid biogenesis. Apart from thylakoid ultrastructural defects, the abrogation of the STIC function leads to the massive induc-tion of an as yet uncharacterized gene, HINAS1, potentially mediated by a hormone sig-nal originating from the chloroplast. How such a signal could indirectly suppress the defects of tic40 mutants is discussed. It is further shown that the alb3 alb4 double mutants are smaller and accumulate less pigments than the alb3 single mutants, and that the chloroplast ultrastructure is further deteriorated in the double mutant compared to alb3. Similarly, the cpftsy mutant and the cpsrp54 cpsrp43 double mutants show also a more severe phenotype in the alb4 back-ground. These findings are backed up by the detection of weak but specific interactions of ALB4 with ALB3 and both cpSRP components, suggesting that the functions of ALB4 and its paralogue ALB3 overlap partially, and that thus both components likely contribute differentially but synergistically to the same process of protein insertion into the thylakoids via the chloroplast signal recognition particle pathway.