Ambient light conditions affect development throughout the plant life cycle, including seed germination, seedling development and the induction of flowering. In the model plant Arabidopsis, the COP1-SPA ubiquitin ligase complex plays a central role in suppressing light signaling in darkness. The COP1-SPA complex targets positively acting factors like HY5, a protein necessary for normal seedling development in the light, several photoreceptors and the flowering time regulator CONSTANS for degradation via the 26S proteasome. Therefore, one of the major functions of the light signal transduction pathways is the inactivation of the COP1-SPA complex. While COP1 is a single copy gene, the SPA proteins are encoded by four different loci (SPA1-SPA4). All SPA proteins have redundant, but also distinct functions in regulating plant development. SPA1 and SPA2 are the key regulators that suppress photomorphogenesis in dark-grown seedlings. Over-stimulation in light-grown seedlings is primarily prevented by SPA1, and to a minor extent, also by SPA3 and SPA4. SPA2, in contrast has only negligible function in the light. SPA1 is sufficient for repressing flowering under non-inductive short-day conditions. Here, I show that distinct functions of the SPA genes partially correlate with their distinct gene expression patterns. RNA gel blot-analysis revealed that the expression of SPA1, SPA3 and SPA4, but not that of SPA2, is positively influenced by light of different wavelengths. All main photoreceptors contribute to the up-regulation of these SPA transcripts, implying that photoreceptors initiate a negative feedback regulation, which might protect plants from over-stimulation by light. GUS reporter gene experiments show that SPA genes exhibit somewhat distinct tissue-specific expression patterns, which might be important for tissue specific regulation of COP1-SPA targets. However, differences in SPA gene expression cannot account for all distinct SPA gene functions. Promoter-swap experiments with SPA1, SPA2 and SPA4 show that all SPA proteins are potent repressors in dark-grown seedlings. SPA1 and SPA4 also act as repressor in the light. SPA2, however, can never act as a repressor in the light, not even when it is expressed from the strong light-induced SPA1 promoter. These results show that SPA proteins themselves feature properties that determine characteristic SPA protein functions. All SPA proteins feature a characteristic domain structure with a C-terminal WD-repeat, a central coiled-coil domain and a less well-conserved N-terminus that includes a kinase-like motif. The WD-repeat domain and the coiled-coil domain are essential for formation of the COP1-SPA complex as well as interactions with various ubiquitination targets. In contrast, the function of the N-terminal domain is unknown. Aiming to determine the importance of the N-terminal domain of SPA1, I conducted a structure-function analysis. While the N-terminal domain of SPA1 is dispensable for SPA1 function in the seedling stage, this domain is required for SPA1-mediated repression of flowering in non-inductive short-day conditions. These results indicate, that the SPA1 N-terminal domain can full-fill an essential function
