Forwarding with iptables

I have a fedora machine that contains 1 interface and I would like to
be able to have it accept a connection for port X and perform port
forwarding to another IP on a seperate port. I know I can do this with
SSH port forwarding; however, I am limited to not being able to SSH
tunnel the connection.

How would I create an iptables entry to do this? An example would be:

Fedora (port 119 - NNTP)
| accepts connection
v
ISP NNTP (port 119)

Because my ISP requires the connection to originate from it's network,
this is one reason I would like to mask the connection if that is
possible.