From Eterna Wiki
GU bonds are bonds between Guanine and Uracil. Often referred to as 'weak bonds', GU bonds are the weakest of the three bonds that can be made in eteRNA (GU, AU, and GC in ascending order). However, GU bonds are suspected to play an important role in the RNA by acting as recognition sites for interactions with other protein biosynthesis components.
GU bonds are the subject matter of the very popular 'watch theory'.