PscA

PscA is the core reaction-center protein found in a subset of photosynthetic bacteria, particularly green sulfur bacteria (Chlorobi) and green non-sulfur bacteria (Chloroflexi). The protein is the PscA subunit of a type I reaction center and is homologous to the PsaA and PsaB subunits of Photosystem I in cyanobacteria and plants. PscA forms the central scaffold for light-induced charge separation by binding and coordinating a set of pigments, including bacteriochlorophylls and associated cofactors, and it interacts with peripheral antenna proteins to funnel excitation energy to the primary electron donor.

Structurally, PscA is an integral membrane protein with multiple transmembrane helices (the number is typically in the vicinity of 10–12) that cradle the core pigments and cofactors. The exact subunit composition and stoichiometry of the reaction-center complex vary among species, but a PscA-centered core often associates with additional core proteins such as PscB and PscC, which help organize the electron-transfer chain and cofactors.

Genetic and evolutionary context shows that pscA genes are distributed across several lineages of phototrophic bacteria and are commonly found within photosynthesis gene clusters alongside genes encoding accessory RC components and light-harvesting elements. Phylogenetic analyses support a shared ancestry between PscA and the Photosystem I core subunits PsaA/PsaB, reflecting deep evolutionary relationships among photosynthetic reaction centers.

Research on PscA contributes to understanding the diversity of photosynthetic reaction centers and the evolution of electron-transfer pathways in bacteria. Studies of its structure and function illuminate how different lineages implement solar-to-chemical energy conversion using type I reaction centers.