* Dollar under pressure at five-week low ahead of Federal Reserve * European shares boosted by M&A activity * Nikkei plumbs four-week lows, other Asian markets also softer * Data, Fed, ECB, Bank of England meetings pose hurdles this week By Marc Jones LONDON, July 29 (Reuters) - Expectations the Federal Reserve will reaffirm its pledge to keep U.S. interest rates near zero left the dollar at a five-week low on Monday, while concerns about China's stuttering economy pressured commodity markets. The Federal Reserve, the European Central Bank and the Bank of England meet this week. All are expected to repeat or refine their "forward guidance" that borrowing costs will remain extraordinarily low as long as growth is sub-par and inflation poses no threat. The Fed will be most closely scrutinised, having signalled plans to begin phasing out its money-printing stimulus this year. Most economists are eyeing a September start but markets have scaled back views of any aggressive changes. It is a shift that has seen the dollar give back three-quarters of June's 5 percent gain and the greenback remained under pressure as U.S trading began, having earlier hit a five-week low against a basket of currencies and one-month low against the yen. "The dollar faces a lot of key event risk in the week ahead with the release of the U.S. Q2 GDP report and the latest FOMC policy meeting on Wednesday, followed by the release of the U.S. employment report for July on Friday," said Lee Hardman, currency strategist at Bank of Tokyo Mitsubishi. Wall Street was expected to open lower with falls of between 0.15-0.25 percent seen for the S&P 500 and Dow Jones Industrial Average. European share markets remained buoyant as two more giant merger deals, this time in the media and pharmaceuticals sectors, added to a flurry of M&A activity in recent weeks. The FTSEurofirst 300 index of top European shares was off its peak as the U.S. open neared, but was still up 0.3 percent, with London, Frankfurt and Paris all 0.2-0.3 percent higher. European shares have started to outperform those in other major regions in recent weeks and the splurge in corporate deals has added to signs that business confidence and economic growth may be finally picking up after almost 18 months of recession. "These big M&A deals are a big boost for the market, although the buzz is usually short-lived," said Philippe de Vandiere, analyst at Altedia Investment Consulting in Paris. DELICATE CHINA The lower dollar helped commodities markets gradually erase falls but both oil and copper were at or near three-week lows. Concerns about demand weighed on crude, while nervousness ahead of Chinese manufacturing data on Thursday hit copper. With investors bracing for another round of disappointing economic news from the world's No. 2 economy, Asian markets had been generally weaker earlier. Japan's Nikkei dropped 3.3 percent to hit a four-week low as those jitters were compounded by a stronger yen, which is negative for the country's exporters, and concerns that plans to increase the country's sales tax - Japan's most significant fiscal reform in years - could be watered down. "A sense of caution is looming in the market, especially because investors are worried about a slowdown in the Chinese economy. And when they see a risk in Asia, they tend to buy the yen, and the Japanese market is hit by that," said Kyoya Okazawa, head of global equities at BNP Paribas. CENTRAL BANK CAUTION On Wall Street, investors may use the uncertainty over central bank stimulus to cash in recent gains. With just three trading days left, the S&P 500 is set to post its best month since October 2011. The Nasdaq's advance makes July so far the best month in a year and a half. In debt markets, euro zone periphery bonds eased but U.S. Treasuries and German Bunds were little changed in thin trade as investors refrained from placing big bets before this week's monetary policy decisions and data. Meanwhile, gold seen as a hedge against central bank money-printing, shook off a subdued start, edging up to $1,333 an ounce as it extended the 11.5 percent rally it has enjoyed since the Fed has softened its stimulus withdrawal message. "This week offers several opportunities to test whether the rally can be sustained, with the central banks' meetings and the U.S. non-farm payrolls on Friday," Saxo Bank senior manager Ole Hansen said.