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. Substrate preference for cystathionine synthesis is O-phospho-L-homoserine (OPH) > O(4)-succinyl-L-homoserine (OSH) >> O-acetyl-L-homoserine (OAH). Is able, at extremely low rate, to catalyze a gamma-elimination of OPH in the absence of cysteine to produce inorganic phosphate (Pi), 2-oxobutanoate and ammonia. L-cysteine + O-succinyl-L-homoserine = H(+) + L,L-cystathionine + succinate Binds 1 pyridoxal 5'-phosphate per subunit. Inhibited by propargylglycine. kcat is 30 sec(-1) with L-cysteine as substrate. Optimum pH is 7.5. Amino-acid biosynthesis; L-methionine biosynthesis via de novo pathway; L-cystathionine from O-succinyl-L-homoserine: step 1/1. Down-regulated by methionine. A DNA region of the first exon coding for a conserved motif of 11 amino acids in CGS1 (positions 77-87) is required for post-transcriptional autoregulation and acts in cis to down-regulate its own mRNA stability in response to excess methionine. This conserved motif is dispensable for CGS enzymatic activity and only found in plant CGSs (PubMed:10558994, PubMed:12121993). It is unclear whether the transit peptide cleavage site is between Phe-68 and Val-69 (PubMed:9531508) or Ala-90 and Ala-91 (PubMed:25146485). Belongs to the trans-sulfuration enzymes family.