May 19 (Reuters) -

Wall Street's main indexes opened lower on Monday as

Treasury yields rose after Moody's downgraded the U.S. sovereign

rating, sharpening focus on its mounting debt.

The Dow Jones Industrial Average fell 111.9 points,

or 0.26%, at the open to 42542.81. The S&P 500 fell 55.5

points, or 0.93%, at the open to 5902.88​, while the Nasdaq

Composite dropped 273.7 points, or 1.42%, to 18937.417

at the opening bell.