Shares of Infosys tumbled over 10 per cent after the company today lowered its revenue guidance in dollar terms for 2012-13, dragging down the BSE benchmark Sensex.The Sensex plunged by 205 points to trade at 17,283 in late morning deals. The 30-share barometer tanked over 244 points in opening trade today. Similarly, the Nifty at the National Stock Exchange declined by 50 points to 5,255 a short while ago.