Billionaire activist investor Carl Icahn announced Wednesday that he is putting $150 million into a super PAC to back legislation to prevent companies from moving their headquarters out of the U.S. through corporate inversions.

In a letter published on his site, Icahn lent his support to tax reform favored by House Ways and Means Committee Chairman Paul Ryan and senators Chuck Schumer, D-N.Y., and Rob Portman, R-Ohio, intended to eliminate inversions.

Right now "is by far the worst time imaginable to have an exodus of hundreds of companies," Icahn wrote, warning that the U.S. economy is "extremely fragile."

Icahn, who is reportedly worth upwards of $20 billion, made his fortune through forcing companies to restructure to become more profitable.

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He expressed concern about inversions, which are tax maneuvers in which U.S. businesses buy a smaller company in a low-tax jurisdiction, and then move the headquarters of the combined company into that country to lower their tax bill.

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