You'll need a 52 mm to 43 mm step down ring. I don't think you'll find one. What I would do would be to get a 52mm to 49mm step down ring and then a 49mm to 43mm step down ring to get there in two steps.
You can order them from:
if you can't find them locally.
I hope this isn't a wide-angle lens. I believe you'd have too much vignetting, if so.