Based on the way the 2 photos are captioned, I think the OP meant "These were taken with the DA55-300 lens at 300mm *and* with a Promaster 100mm macro lens." 2 different lenses, ruling out a dirty lens.
Certainly looks like hair or something thread-like. Anyone have experience with hair on the sensor? If my understanding of optics is correct a hair on the sensor would probably leave a sharper, in-focus mark. The hair in the photos looks blurry and therefore somewhere in front of the focal plane.
Doing another cleaning is probably needed but first check to see if you can't spot the dirt or whatever it is. Work in a semi-dark room so your eyes dilate. Shine a flashlight into the camera and look at the mirror, sensor, mount, and space in-between. The hair or thread might be visible.
Last edited by DeadJohn; 07-01-2011 at 05:30 PM.