Biochemical Studies on 2-Deoxy-scyllo-inosose, an Early Intermediate in the Biosynthesis of 2-Deoxystreptamine. VII. Substrate Specificity of 2-Deoxy-scyllo-inosose Synthase, the Starter Enzyme for 2-Deoxystreptamine Biosynthesis, toward Deoxyglucose-6-phosphates and Proposed Mechanism