Leases will be made only to persons who are citizens of the United States or have declared their intention to become so, or corporations which are organized under the laws of the United States or one of the States or Territories: Provided, That no lease, assignment thereof, or interest therein will be approved to any employee or employees of the United States Government, whether connected with the Bureau or otherwise, and no employee of the Interior Department shall be permitted to acquire any interest in such leases by ownership of stock in corporations having leases or in any other manner.