Definition on BMBCB http://www.biochem.northwestern.edu/holmgren/Glossary/Definitions/Def-P/polar_mutation.html
"Microbial Physiology" – Google Books https://books.google.com/books?id=DdTIl6-1UzgC&dq=%22Polar+mutation%22&pg=PA146
"Fundamental bacterial genetics" – Google Books https://books.google.com/books?id=2iazWp_GVtUC&dq=%22Polar+mutation%22&pg=PA274