5 USC 3345 – Acting officer
(a) If an officer of an Executive agency (including the Executive Office of the President, and other than the Government Accountability Office) whose appointment to office is required to be made by the President, by and with the advice and consent of the Senate, dies, resigns, or is otherwise unable to perform the functions and duties of the office—
(1) the first assistant to the office of such officer shall perform the functions and duties of the office temporarily in an acting capacity subject to the time limitations of section 3346;
(2) notwithstanding paragraph (1), the President (and only the President) may direct a person who serves in an office for which appointment is required to be made by the President, by and with the advice and consent of the Senate, to perform the functions and duties of the vacant office temporarily in an acting capacity subject to the time limitations of section 3346; or
(3) notwithstanding paragraph (1), the President (and only the President) may direct an officer or employee of such Executive agency to perform the functions and duties of the vacant office temporarily in an acting capacity, subject to the time limitations of section 3346, if—
(A) during the 365-day period preceding the date of death, resignation, or beginning of inability to serve of the applicable officer, the officer or employee served in a position in such agency for not less than 90 days; and
(B) the rate of pay for the position described under subparagraph (A) is equal to or greater than the minimum rate of pay payable for a position at GS-15 of the General Schedule.
(b)(1) Notwithstanding subsection (a)(1), a person may not serve as an acting officer for an office under this section, if—
(A) during the 365-day period preceding the date of the death, resignation, or beginning of inability to serve, such person—
(i) did not serve in the position of first assistant to the office of such officer; or
(ii) served in the position of first assistant to the office of such officer for less than 90 days; and
(B) the President submits a nomination of such person to the Senate for appointment to such office.
(2) Paragraph (1) shall not apply to any person if—
(A) such person is serving as the first assistant to the office of an officer described under subsection (a);
(B) the office of such first assistant is an office for which appointment is required to be made by the President, by and with the advice and consent of the Senate; and
(C) the Senate has approved the appointment of such person to such office.
(c)(1) Notwithstanding subsection (a)(1), the President (and only the President) may direct an officer who is nominated by the President for reappointment for an additional term to the same office in an Executive department without a break in service, to continue to serve in that office subject to the time limitations in section 3346, until such time as the Senate has acted to confirm or reject the nomination, notwithstanding adjournment sine die.
(2) For purposes of this section and sections 3346, 3347, 3348, 3349, 3349a, and 3349d, the expiration of a term of office is an inability to perform the functions and duties of such office.