* 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
Expectations that the Federal Reserve will reaffirm its commitment to keeping U.S. interest rates near zero left kept 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 ultra accommodative policy this year. Most economists are eyeing a September start but markets have scaled back views of any aggressive changes.
The dollar, which shed 1.2 percent last week for its third straight weekly loss, remained under pressure at 81.540 by mid-morning in Europe, having earlier hit a 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 though 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 up 0.4 percent at 1,212 points by 0745 GMT, with London, Frankfurt and Paris 0.4-0.5 percent higher.
Commodities markets were mostly softer, with both oil and copper 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.
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.
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, German Bund futures edged back into negative territory in thin trade and euro zone periphery bonds eased, but investors refrained from placing big bets before this week's monetary policy decisions and data.