RIO DE JANEIRO (Reuters) -The chief executive of Brazil's Petrobras offered to step down, the state-run oil company said on Tuesday, as the government lined up a former regulator for the role, sending shares sliding as investors braced for political interference. Brazilian President Luiz Inacio Lula da Silva will replace the outgoing CEO Jean Paul Prates with Magda Chambriard, the former head of Brazilian oil and gas regulator ANP, according to a securities filing citing the Mines and Energy Ministry. New York-listed shares of Petrobras fell more than 6% in after-hours trading as news broke that Prates was headed for the exit.