U.S. President Donald Trump says he is nominating a deputy to Energy Secretary Rick Perry to replace him at the helm of the Energy Department.

'I am pleased to nominate Deputy Secretary Dan Brouillette to be the new Secretary of Energy," Trump tweeted on October 18, the day after he announced that Perry -- a key player in the controversy over Trump's dealings with Ukraine -- would leave his position.

The U.S. president wrote that Brouillette's experience in the area is 'unparalleled' and called him a 'total professional.'

Trump also thanked Perry for "the outstanding job he has done," and said he would leave his post "at the end of the year to pursue other interests."

The U.S. administration has gone through a high level of turnover of top officials and cabinet members during Trump's presidency, which began in January 2017.

Democrats in the U.S. House of Representatives have issued a subpoena to Perry as part of an impeachment inquiry into dealings with Ukraine.

The subpoena asks to Perry to provide documents related to a Ukrainian state-owned oil and gas conglomerate, Naftogaz, as well as his involvement in a July 25 call between Trump and Ukrainian President Volodymyr Zelenskiy.

In a Fox News interview on October 18, Perry said his planned departure had "absolutely nothing to do" with the controversy over the Trump administration's handling of Ukraine, and that he was looking to return to Texas for 'the next adventure in life.'

Asked whether he would cooperate with the impeachment inquiry, Perry deferred to his legal team.

'Our general counsel is taking a look at that right now and when they send their answer at the end of the day, whatever their decision will be, I'll follow that,' he said.

Trump has said Perry encouraged him to make the July 25 call, in which the U.S. president pressed Ukraine to investigate his Democratic rival Joe Biden and his son, who was on the board of a Ukrainian gas company

Perry, a former governor of Texas, who faced Trump in the 2016 Republican primary elections, has previously denied published reports that he was planning to resign in November.

With reporting by AP and Reuters

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