prepend a character to the lines in a region

[B. T. Raven]

If I want to respond to an email where (for whatever reason) the sender's
text hasn't been quoted, I copy paste it into emacs and do a search
replace with this regular expression:

C-M-% \(.+\) RET >\1  RET

This works but the region is repainted before each replacement. Is there a
built in command that can accomplish this? M-; is almost right but it
might not be if I had a defined syntax and I actually wanted to use it for
commenting out a region.

Ed.