(a) Membership in the elected class of the retirement system is limited to:
(1) persons who hold state offices that are normally filled by statewide election and that are not included in the coverage of the Judicial Retirement System of Texas Plan One or the Judicial Retirement System of Texas Plan Two;
(2) members of the legislature; and
(3) district and criminal district attorneys, to the extent that they receive salaries from the state general revenue fund.
(b) Membership in the elected class is optional.
(c) An eligible person becomes a member of the elected class by filing a notice of intention to become a member with the board of trustees on a form prescribed by the board.