Catalyzes the methylthiolation of an aspartic acid residue of ribosomal protein S12. [sulfur carrier]-SH + AH2 + L-aspartate(89)-[ribosomal protein uS12]-hydrogen + 2 S-adenosyl-L-methionine = 3-methylsulfanyl-L-aspartate(89)-[ribosomal protein uS12]-hydrogen + 5'-deoxyadenosine + [sulfur carrier]-H + A + 2 H(+) + L-methionine + S-adenosyl-L-homocysteine Binds 2 [4Fe-4S] clusters. One cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine. Belongs to the methylthiotransferase family. RimO subfamily.