Catalyzes the first committed step of methionine (Met) biosynthesis. Catalyzes the formation of L-cystathionine from homoserine esters and L-cysteine, via a gamma-replacement reaction. L-cysteine + O-succinyl-L-homoserine = H(+) + L,L-cystathionine + succinate Binds 1 pyridoxal 5'-phosphate per subunit. Amino-acid biosynthesis; L-methionine biosynthesis via de novo pathway; L-cystathionine from O-succinyl-L-homoserine: step 1/1. Belongs to the trans-sulfuration enzymes family. Truncated N-terminus.