Squalene synthase in plants – Functional intricacy and evolutionary divergence while retaining a core catalytic structure

Squalene is the crucial intermediate for the biosynthesis of many bioactive triterpenoids, such as phytosterol in plants or cholesterol in animals. Squalene synthase (SQS) is the essential gene of the squalene biosynthetic pathway, which catalyzes the head-to-head condensation of two farnesyl pyrophosphate or farnesyl diphosphate (FPP) molecules in a two-step reaction and formation of linear C₃₀ squalene. SQS ubiquitously occurs in all eukaryotic organisms. However, the activity of this gene varies significantly, leading to diverse squalene content in plants. The present study focused on the variation in the expression landscape of SQS gene copies with varying evolutionary backgrounds. Afterward, a reflection of the sequence divergence on the catalytic structure of the protein was examined. The genome-scale mining of the SQS homologs revealed varying degrees of duplication events, sequence evolution of the gene sequence itself, and the adjoining regulatory architecture. Contrasting expressional patterns and the regulatory modules pinpoint the importance of transcriptional regulation of this essential gene. Three-dimensional organizations of SQS from diverse evolutionary taxa and their consensus structures enlightened the conservation of critical catalytic domains, nonetheless divergence in the majority of the protein. As a whole, the outputs of this study provide some valuable insights for understanding the functional regulation of SQS under different tissues and environments.