A possible mechanistic insight on how Compromised Hydrolysis of Triacylglycerol 7 (CHT7) restrains the involvement of it's DNA binding CXC domain from quiescence repression

CHT7 is a regulator of quiescence repression and TAG degradation between the nitrogen deprived and the nitrogen replenished states in Chlamydomonas reinhardtii. Initially it was thought that the CHT7s repression activity is managed by its DNA binding CXC domain which is a tandem repeat of two cysteine rich subdomains. Later, it was found that the CXC (CHT7_CXC) domain is effectively dispensable for CHT7s activities. Rather, CHT7s predicted protein binding domains are proposed to be involved in gene regulation activities by binding through other repressors in the cell. Yet, it remains unclear why and how CHT7 manages to refrain its own CXC domain from participating in any transcriptional activities. The question becomes more intriguing, because CXC binding regions are available in promoter regions of some of the misregulated genes in the CHT7 mutant (cht7). Through the combination of biophysical experiments and molecular dynamics approaches, we have studied the DNA recognition behavior of CHT7_CXC. The results show that CHT7_CXC domain is highly selective towards DNA sequences and this selectivity is imparted due to the differential binding abilities of the CXC subdomains. Further, to understand if the case is - that CXC looses its DNA binding capabilities in the vicinity of other repressor molecules, we carried out CHT7_CXCs DNA binding stability test by simulating the spatial constraint conditions using the AsLOV2- CXC fusion. Our test results show limited ability of CHT7_CXC to withstand steric forces and provide insights to why and how algal cells may hold back CHT7_CXCs indulgence in quiescence repression. SignificanceMicroalgae, under nutrient rich conditions, provide biomass. Whereas, nutrient deprivation leads to accumulation of biofuel feedstock, but cells enter quiescence. Net enhancement in feedstock, therefore relies on the precision of the quiescence regulator. In Chlamydomonas reinhardtii, CHT7 is a central regulator of quiescence. Surprisingly, rather than using its own DNA binding domain (DBD) for the regulatory activities, CHT7 recruits external transcriptional regulators using its non DBDs. To ensure smooth functioning, CHT7s DBD must rapidly switch to inactive form. Modifications in DNA binding profiles of DBDs due to non DBDs are seen in transcription factors of many organisms. The switching mechanism discussed could therefore be a generic approach of timely regulation of individual components of the complex transcriptional machineries.