Assistant or deputy commissioners of the Bureau of Indian Affairs, in the Department of the Interior, shall be appointed by the Secretary of the Interior, subject to the civil-service laws and chapter 51 and subchapter III of chapter 53 of title 5. Appointments to these positions shall be considered as made under the authority of section 3101 of title 5. Assistant and deputy commissioners so appointed shall be authorized to sign such letters, papers, and documents and to perform such other duties as may be directed by the commissioner of the Bureau of Indian Affairs. The Secretary may designate for the Bureau of Indian Affairs an assistant or deputy commissioner, who shall be authorized to perform the duties of the commissioner in case of the death, resignation, absence, or sickness of the commissioner.