Watch the Shuffle 12 Bar Variation 1 online guitar lesson by Corey Congilio from Rhythm Factory: Texas Blues
We're in the key of A here and I believe that rhythm parts have a melody attached to them as well. To me, a melody is anything you can sing or hum to. Many of these rhythm parts will provide that. You don't have to use all three examples here but, give it a shot and play them however you'd like to!