activating digitalpin by RTS command.

I want to activate a optoisolator with my Arduino when my software send an RTS signal to the board. This is to control a transmitter and to activate it, the software send an RTS signal to a com port at 1200 baud.

In fact the project was using a simple usb to RS232 FTDI cable, and I had to connect the optoisolator to the RTS pin on the cable but as I have two unused Arduino board (one Arduino Deciemilia and one Seeduino 1.1) I prefer using what I already have.

Some one have a clue on how to do it?

p.s. I have send the same question on the french forum, but wanted to put it on both language on the site. Thanks, have a nice day.