Credit ratings for the main arteries of the U.S. financial system -- the Depository Trust Co, National Securities Clearing Corp, Fixed Income Clearing Corp and the Options Clearing Corp -- were cut one notch to AA-plus by Standard & Poor's Ratings Services Monday.

These institutions, previously rated AAA by S&P, clear and process trades and are crucial to the daily workings of the U.S. financial markets.

S&P in a statement said the downgrades were due to its lowering of the U.S. sovereign credit rating late on Friday, a decision that is prompting the credit agency to review and in some cases slice the ratings of a host of entities whose financial health depends heavily on the federal government...