Aryeh Bourkoff, UBS's head of investment banking for the Americas, and a 13-year veteran of the firm resigned - effective immediately.  He will "pursue other opportunities," according to a firm internal memo. Mr. Bourkogff's interim replacement is Steve Cummings, a former Wachovia executive whom UBS hired last year as chairman of the Americas investment banking division. Bourkoff's UBS Career. A 13-year veteran of the firm, Mr. Bourkoff began his career at UBS as an analyst covering technology, media and telecommunications.  He switched over to the banking side in 2007, becoming the head of the firm’s telecom and media team and one of its most prominent deal-makers.  His reign as Americas head of investment banking lasted less than one year. This departure makes the latest in a wave of executive changes at UBS and its investment bank.  Just last month, the bank hired Andrea Orcel as co-head of investment banking.  He came over from Bank of America, where he had been chairman of global banking and markets. For the internal memo and further details, go to:  [Dealbook, 4/17/12].