June 18 (Reuters) - Nippon Steel's $14.9 billion acquisition of U.S. Steel closed on Wednesday, the companies said, confirming an unusual degree of power for the Trump administration after the Japanese company's 18-month struggle to close the purchase.

Under the deal terms, Nippon bought 100% of U.S. Steel shares at $55 per share, as it first laid out in its December 2023 offer for the well-known and struggling steelmaker.

The filing also discloses details of a national security agreement inked with the Trump administration, which gives President Donald Trump the authority to name a board member as well as a non-economic golden share.

That share gives the U.S. government veto authority over a raft of corporate decisions, from idling plants to cutting production capacity and moving jobs overseas, as previewed in a weekend social media post by Commerce Secretary Howard Lutnick.

The measures, represent an unusual level of control conceded by the companies to the government to save the deal, after a rocky path to approval spurred by high-level political opposition.

The inclusion of the golden share to win approval from the Committee on Foreign Investment in the U.S., which scrutinizes foreign investment for national security risks, could drive overseas investors away from U.S. companies, national security lawyers said on Monday.

After the United Steelworkers union came out against the deal last year, both then-President Joe Biden, a Democrat, and Trump, a Republican, expressed their opposition as they sought to woo voters in Pennsylvania, a key swing state, in the presidential election campaign.

Shortly before leaving office in January, Biden blocked the deal on national security grounds, prompting lawsuits by the companies, which argued the national security review they received was biased. The Biden White House disputed the charge.

The steel companies saw a new opportunity in the Trump administration, which opened a fresh 45-day national security review into the proposed merger in April.