Produces L-cysteine from O-acetyl-L-serine and hydrogen sulfide. Can also use pyrazole and 3,4-dihydroxypyridine instead of the hydrogen sulfide to produce two plant specific non-protein amino acids beta-pyrazolylalanine and L-mimosine. hydrogen sulfide + O-acetyl-L-serine = acetate + L-cysteine O-acetyl-L-serine + pyrazole = 3-(pyrazol-1-yl)-L-alanine + acetate + H(+) L-serine + pyrazole = 3-(pyrazol-1-yl)-L-alanine + H2O 3,4-dihydroxypyridine + O-acetyl-L-serine = 3-(3,4-dihydroxypyridin-1-yl)-L-alanine + acetate + H(+) Amino-acid biosynthesis; L-cysteine biosynthesis; L-cysteine from L-serine: step 2/2. Homodimer. Belongs to the cysteine synthase/cystathionine beta-synthase family.