how to REINDEX pg_database on

I'm having a problem with Postgres 7.4 on RHEL 4 (Nahant Update 8).
It seems that pg_database disappeared some time ago (I think before
my oldest backups, unfortunately).

I searched and found some mention of REINDEX but I'm not sure if I am
using it correctly since this is a system catalog. I tried this:

postgres -P -D /usr/local/databases/pgsql/data pg_database
FATAL: database "pg_database" does not exist

I also tried connecting to a working database to run the command:

backend> REINDEX DATABASE pg_database
ERROR: can only reindex the currently open database

However, I have at least one working database:

postgres -P -D /var/lib/pgsql/data pg_bmap_repo

POSTGRES backend interactive interface
$Revision: 1.375.2.5 $ $Date: 2005/12/14 17:07:00 $

backend> select * from fMRIStudy;
1: _oid (typeid = 23, len = 4, typmod = -1, byval = t)
2: imaging_study (typeid = 23, len = 4, typmod = -1, byval = t)
3: function (typeid = 1043, len = -1, typmod = 24, byval = f)
4: mrseries (typeid = 23, len = 4, typmod = -1, byval = t)
5: description (typeid = 1043, len = -1, typmod = 104, byval = f)

Thanks for any help!