Catalyzes the dehydration of methylthioribulose-1-phosphate (MTRu-1-P) into 2,3-diketo-5-methylthiopentyl-1-phosphate (DK-MTP-1-P). S-methyl-5-thio-D-ribulose 1-phosphate = 5-methylsulfanyl-2,3-dioxopentyl phosphate + H2O Binds 1 zinc ion per subunit. Amino-acid biosynthesis; L-methionine biosynthesis via salvage pathway; L-methionine from S-methyl-5-thio-alpha-D-ribose 1-phosphate: step 2/6. Belongs to the aldolase class II family. MtnB subfamily.