Component of the sulfite reductase complex that catalyzes the 6-electron reduction of sulfite to sulfide. This is one of several activities required for the biosynthesis of L-cysteine from sulfate. The flavoprotein component catalyzes the electron flow from NADPH -> FAD -> FMN to the hemoprotein component. 3 H2O + hydrogen sulfide + 3 NADP(+) = 4 H(+) + 3 NADPH + sulfite Binds 1 FAD per subunit. Binds 1 FMN per subunit. Sulfur metabolism; hydrogen sulfide biosynthesis; hydrogen sulfide from sulfite (NADPH route): step 1/1. Alpha(8)-beta(8). The alpha component is a flavoprotein, the beta component is a hemoprotein. Belongs to the NADPH-dependent sulphite reductase flavoprotein subunit CysJ family. In the N-terminal section; belongs to the flavodoxin family. In the C-terminal section; belongs to the flavoprotein pyridine nucleotide cytochrome reductase family.