She fills the job vacated by Cummings when he was promoted last year, and will be based in New York.
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In her new role, Deignan will oversee the bank's equity capital markets operations in North and Latin America.
Before UBS, she spent 11 years at Merrill Lynch, now owned by Bank of America, in a variety of roles.
(Reporting by Joe Rauch, editing by Gerald E. McCormick and Lisa Von Ahn)