Full form of RuBP is Ribulose 1,5 biphosphate. RuBP is the first acceptor of atmospheric C02 during dark
reaction of photosynthesis. The reaction is called carboxylation.
6RuBP + 6Co2 + 6H2o ------» Unstable 6C compound ------> 3PGA
RuBP is regenerated during final formation of sugar molecule.