* Looking at possible breach of ad hoc publicity directive
* Possible breach of corporate governance directive
* Investigation to continue for an indefinite period
* Looking at possible breaches during 2007 and 2008
(Adds UBS no comment, shares, analyst comment, background)
ZURICH, Sept. 18 (Reuters) - Switzerland's stock exchange is investigating UBS for possible breaches of rules regarding publication of price sensitive information and information on its board, it said on Friday. SIX Exchange Regulation said it had started the investigation into possible breaches of the ad hoc publicity directive -- used to regulate potentially price-sensitive information -- by UBS from 2007 to the end of 2008.
SIX Exchange Regulation, which monitors and enforces issuers' obligations for the SIX Swiss Exchange, is also looking into possible breaches of the corporate governance directive in connection with the UBS 2008 annual report.
UBS declined to comment.
Shares in UBS were trading 1.8 percent lower at 18.98 Swiss by 0820 GMT, underperforming the Dow Jones Stoxx European banks index which was 0.9 percent lower.
The exchange's corporate governance directive requires issuers to disclose important information on their board and senior management or to give substantial reasons why this information is not disclosed.
UBS, which is struggling to rebuild its reputation after a U.S. government tax evasion probe and after chalking up huge losses in the credit crisis, revised its 2008 accounts twice, once in March and again in May.
A spokesman for the SIX Exchange declined to comment on whether that was the reason for the investigation, adding he could not give any further details.
"It is normal that there is an investigation after they gave three different sets of numbers," said Vontobel analyst Teresa Nielsen. "I don't believe this will have any material impact on UBS' earnings."
In March, UBS said it had revised up its 2008 net loss by 1.2 billion francs to include a big U.S. tax fine and more writedowns from the 19.7 billion francs it originally reported, already the biggest loss in Swiss corporate history.
The other additional charges that increased the 2008 loss related to the Swiss National Bank's (SNB) valuation of around $7.8 billion of securities not yet transferred to an SNB stability fund, dedicated to mopping up UBS's toxic assets.
In May, UBS restated its 2008 accounts again, raising its full year loss by a further 405 million Swiss francs to correct accounting errors. It also reduced equity and equity attributable to UBS shareholders by 269 million francs.
The investigatory proceedings will continue for an indefinite period, the SIX Exchange said.
"SIX Exchange will announce its findings, although no information will be provided while the proceedings are ongoing," it said. (Reporting by Katie Reid and Emma Thomasson; Editing by Jon Loades-Carter)