AnswerX1

From 2006.igem.org

See link for more info.: [http://www.pubmedcentral.gov/picrender.fcgi?artid=413488&blobtype=pdf]

HU is the only protein in E. coli that can promote invertasome formation w/ short DNA lengths between the enhancer and recombination sites. However, the presence of certain polyamines and a protein activity present in HeLa nuclear extracts can efficiently substitute for HU in invertasome assembly.