NEW YORK Stock index futures pointed to a weak start for Wall Street on Wednesday, with futures for the S&P 500, Dow Jones futures and Nasdaq futures down 0.2 to 0.4 percent by 5:06 a.m. EDT.

* Caution over the euro zone debt crisis resurfaced after Moody's became the first ratings agency to cut the credit rating for Portugal by four notches to non-investment grade, known as junk, warning the country may need a second round of rescue funds before it can return to capital markets.

* The euro lost ground against the dollar and the Swiss franc on euro zone debt worries and peripheral euro zone banking stocks fell across the board.

* U.S. stocks ended mainly flat in thin trade on Tuesday, as investors paused after last week's rally.

* The ISM non-manufacturing index, due at 1400 GMT, is expected to show a slight dip to 54.0 in June from 54.6, according to a Reuters poll, the second-lowest reading since last August.

* Other data scheduled for release include the Challenger planned layoffs for June at 1130 GMT and weekly mortgage index numbers at 1100 GMT.

* In company news, Berkshire Hathaway Inc has joined the group bidding for Citigroup's consumer lending unit OneMain, formerly known as CitiFinancial, the Wall Street Journal said, citing people familiar with the matter.

* U.S. pipeline safety regulators on Tuesday said Exxon Mobil must make fixes to its ruptured Montana oil pipeline and submit a restart plan before oil can flow again.

* Global passenger airplane market over the next 20 years is seen at $4 trillion, Boeing Co said on Wednesday, adding it expects a market for 33,500 new planes and freighters by 2030.

* In Europe, the FTSEurofirst 300 index <.FTEU3> of top shares was lower in early trade, ending a seven-day winning steak on concerns about the euro zone debt crisis and the possibility of contagion after Moody's cut Portugal's credit rating.

(Reporting by Harpreet Bhal, editing by Jane Merriman)