A note on string interaction on the pp-wave background
  We consider type IIB string interaction on the maximally supersymmetric pp-wave background and discuss how the bosonic symmetries of the background are realized. This analysis shows that there are some interesting differences with respect to the flat-space case and suggests modifications to the existing form of the string vertex. We focus on the zero-mode part which is responsible for some puzzling string predictions about the N=4 SYM side. We show that these puzzles disappear when a symmetry preserving string interaction is used. 
Chong-Sun Chu, Valentin V. Khoze, Michela Petrini, Rodolfo Russo,   Alessandro Tanzini