different lock settings for different databases

Hello!
I have a question regarding the lock level for selects:

We are using 3 different databases from our application, these are
configured in our XML in this way:

<descriptor-repository version="1.0" isolation-level="read-comitted">
	<jdbc-connection-descriptor ... for database 1
	<jdbc-connection-descriptor ... for database 2
	<jdbc-connection-descriptor ... for database 3
</descriptor-repository>

What i want now is to set the isolation-level of database 3 to
"read-uncommitted".
How could this done? The jdbc-connection-descriptor tag doesn't support the
attribute isolation-level.

Thanks in advance,
CL