Hi there,
in the manual (http://download1.parallels.com/Plesk/Doc/en-US/online/plesk-administrator-guide/index.htm?fileName=59427.htm) i found the follow entry:
On CentOS 5, RHEL 5, and CloudLinux Plesk installations, you can use MySQL 5.5. It is packaged with Plesk and can be installed...