Europe's worst-hit bank throughout the U.S. mortgage mess, UBS AG (UBS), saw its shares slide further Tuesday morning as the bank shuffled its governance in an effort to appease uneasy investors, kicking four board members to the curb in the process. Stephan Haeringer, Rolf Meyer, Peter Spuhler and Lawrence Weinbach decided to "step down from their posts," UBS said Tuesday. The bank said that it will look to install Italian car magnate Sergio Marchionne, CEO of Fiat, to a new post of senior independent director, while it will look to fill the four now-vacant seats on its board with executives outside the bank. UBS also disclosed new rules it hopes will more clearly separate the board's role from that of company executives -- a nod to previous complaints of investor groups that have claimed that UBS' lack of proper governance gave it a blind spot when it came to the U.S. mortgage mess. The board shuffle at UBS did little, however, to assuage investor concern over what are likely to be multi-billion dollar mortgage losses for the second quarter; shares in the company, which has already absorbed $37 billion in mortgage-led write-downs, were off 5.58 percent in pre-market trading when this story was published. Investors are expecting write-offs as high as CHF 7 billion ($6.9 billion) when the bank reports Q2 results in August, according to a Wall Street Journal story. Shares also took a hit on the news that U.S. officials had taken unprecedented steps in an ongoing investigation over tax evasion by some of the firm's wealthy state-side clientele. Bloomberg News reported Tuesday morning that federal prosecutors had asked a Miami federal judge to let the IRS issue a summons to Zurich-based UBS for the names of U.S.-based customers that used the firm's private banking services to allegedly dodge as much as $20 billion in tax liability. The story says that any implicated individuals could face criminal charges, citing a legal expert on the matter. UBS may be forced to cooperate with the U.S. investigation, the expert said, noting that the Swiss government has been working in cooperation with federal officials in the States. Disclosure: The author held no positions in UBS when this story was originally published. HW reporters and writers follow a strict disclosure policy, the first in the mortgage trade.