5 C.F.R. § 319.402 – Scientific and professional positions.

(a) ST positions are filled without competitive examination under 5 U.S.C. 3325.
(b) ST positions are not subject to the citizenship requirements in 5 CFR part 338, subpart A. Agencies, however, must observe any restrictions on the employment of noncitizens in applicable appropriations acts.
(c) ST employees acquire competitive status immediately upon appointment. They are not required to serve a probationary or trial period.