Who governs the Northern Mariana islands?

According to the CIA World Factbook, the Northern Mariana Islands are a commonwealth of the United States of America. The political system mirrors that of the United States, in that there are three branches of government: the executive, legislative, and judicial branches. The Islands have a locally-elected governor and lieutenant governor that run the executive branch. While American President Barack Obama is the formal chief of state, Governor Benigno R. Fitial, reelected in 2006, acts as the head of the government.