manipulating PS1 in sh

Trying to set PS1 to reflect the current value of a user environment
variable, when running in sh.

Most of my systems I am running in ksh, and my PS is set as follows:

export PS1=`hostname`'.''$ORACLE_SID> '

so that my prompt always shows the current value of $ORACLE_SID. But
it appears that this syntax doesn't yeild the same results with sh.
There, instead of returning the value of $ORACLE_SID, it simply returns
the literal "$ORACLE_SID".

The simple solution is to simply run ksh (my preference) but that
presents certain, uh, political challenges. Is there a simple way of
achieving this same functionality under sh (Solaris 5.9)?

.