l-Threonine　transaldolase　(LTTA)　is　a　putative　serine　hydroxymethyltransferase　(SHMT)　that　can　catalyze　the　trans-aldehyde　reaction　of　l-threonine　and　aldehyde　to　produce　l-threo-β-hydroxy-α-amino　acids　with　excellent　stereoselectivity.　In　the　present　study,　an　l-threonine　transaldolase　from　Pseudomonas　sp.　(PsLTTA)　was　mined　and　expressed　in　Escherichia　coli　BL21　(DE3).　A　substrate　spectrum　assay　indicated　that　PsLTTA　only　consumed　l-threonine　as　the　donor　substrate　and　could　accept　a　wide　range　of　aromatic　aldehydes　as　acceptor　substrates.　Among　these　substrates,　PsLTTA　could　catalyze　p-methylsulfonyl　benzaldehyde　and　l-threonine　to　produce　l-threo-p-methylsulfonylphenylserine　with　a　high　conversion　rate　(74.4%)　and　a　high　de　value　(79.9%).　The　conversion　and　stereoselectivity　of　PsLTTA　were　found　to　be　dramatically　influenced　by　the　concentration　of　the　whole　cell,　the　co-solvent　and　the　reaction　temperature.　Through　conditional　optimization,　l-threo-p-methylsulfonylphenylserine　was　obtained　with　67.1%　conversion　and　a　near-perfect　de　value　(94.5%),　the　highest　stereoselectivity　for　an　l-threo-β-hydroxy-α-amino　acid　so　far　reported　by　enzymatic　synthesis.　Finally,　synthesis　of　l-threo-p-methylsulfonylphenylserine　at　a　100　mL　scale　by　whole-cell　biocatalysis　was　conducted.　This　is　the　first　systematic　report　of　l-threonine　transaldolase　as　a　robust　biocatalyst　for　preparation　of　β-hydroxy-α-amino　acids,　which　can　provide　new　insights　for　β-hydroxy-α-amino　acids　synthesis.　©　The　Royal　Society　of　Chemistry　2019.