When downgrading to MySQL 5.1 from MySQL 5.5, you should keep in mind the following issues relating to features found in MySQL 5.5, but not in MySQL 5.1:
InnoDB. MySQL 5.5 uses
InnoDB Pluginas the built-in version ofInnoDB. MySQL 5.1 includesInnoDB Pluginas of 5.1.38, but as an option that must be enabled explicitly. See Changes in MySQL 5.1.38.