UBS does some quick maths on Japan’s contribution to corporate earnings:
Japan accounts for about 1.5% of S&P 500 Revenues, based on:
␣ Foreign Revenues are ~30% of total S&P 500 revenues
␣ Japan accounted for 4.7% of U.S. exports in 2010
␣ Japan accounted for 5.3% of sales of foreign affiliates of U.S. firms in 2008 (latest available data).
So Japan accounts for about 5% of foreign revenues of U.S. firms or 1.5% of total revenue of the S&P 500 (0.05*0.30 = .015). Profit margins are much lower in Japan than other regions—in 2010E, 3.2% in Japan vs. 7.5% for the world as a whole. (See Jeffrey Palma, Global Profit Picture, March 3, 2011).
As for the specific industries most exposed, this list of exports ranked by amount going to Japan is handy: