If the turnoff sequence aims for FMN1 it should be something like UUCC.
If it aims for FMN2 it should be something like CCUU.
Plus make variations over those themes. Main ingredients are the CC’s. There also sometimes needs to be base additions in the ends. But there is only a smaller window of variations that works for the turnoff regions in either cases. At least for the FMN/MS2 switches.
A good turnoff sequence matches both a region in MS2 and part of one of the FMN sequences.
MS2 turnoff on the left side of the MS2 - should be longer and if long, split in two if the MS2 turnoff has to be at the left side of the MS2. MS2 turnoff on the right side of MS2, should be shorter. (4-5 bases)
Background article: EternaBot report