This question is on behalf of a high school lab I've been donating supplies to. They don't have a -80C freezer. Is it possible to store and maintain competency in a regular -20? This is for DH5alpha cells, btw.

Depends on the protocol used to create them. There are roughly four different basic protocols (and dozens of variations) for CaCl2 competent E. colicells (I assume that is what you meant).

 

With the faster protocols cells tend to lose competence at -20° within a day or two. But unfortunately even with the more elaborate ones they will gradually lose competence. Depending on initial quality it can be easily within a week, or a month at most. Of course, if you transform with pure plasmid even with largely incompetent cells you might score a few cfus.

Thanks charonY