While many are familiar with the federal government's myriad post-employment restrictions, less well known are pre-employment restrictions. One example of a pre-employment restriction is contained in the Lobbyist Disclosure Act which in Section 21 contains the following pre-employment restriction which is codified at 19 U.S.C. § 2171(b)(4):  

(4) A person who has directly represented, aided, or advised a foreign……in any trade negotiation, or trade dispute, with the United States may not be appointed as United States Trade Representative or as a Deputy United States Trade Representative.

This provision has been scrutinized recently in connection with the current U.S. Trade Representative (USTR) nominee and the White House has weighed in with a Clinton-era Department of Justice Opinion stating the provision is unconstitutional…and thus has no legal effect. The nomination is scheduled to be voted on by the Senate Finance Committee on April 6, 2017 and this issue is likely to come up again in connection with the vote. 

In addition to this pre-employment restriction, the USTR and Deputy USTR are subject to a lifetime post-employment band, which is codified at 18 U.S.C. § 207(f)(2):

(f) Restrictions Relating to Foreign Entities.
(2) Special rule for trade representative.-With respect to a person who is the United States Trade Representative or Deputy United States Trade Representative, the restrictions described in paragraph (1) shall apply to representing, aiding, or advising foreign entities at any time after the termination of that person's service as the United States Trade Representative.

Both provisions key off of definitions contained in the Foreign Agents Registration Act and referenced in 18 U.S.C. § 207(f)(3):

(3) Definition.- For purposes of this subsection, the term "foreign entity" means the government of a foreign country as defined in section 1(e) of the Foreign Agents Registration Act of 1938, as amended, or a foreign political party as defined in section 1(f) of that Act.