Catalyzes the condensation of ribulose 5-phosphate with formaldehyde to form 3-hexulose 6-phosphate. D-ribulose 5-phosphate + formaldehyde = D-arabino-hex-3-ulose 6-phosphate One-carbon metabolism; formaldehyde assimilation via RuMP pathway; D-fructose 6-phosphate from D-ribulose 5-phosphate and formaldehyde: step 1/2. Belongs to the HPS/KGPDC family. HPS subfamily.