(a) The board of trustees may, under the standard of care provided by § 840.303, invest and reinvest the retirement system’s assets and may commingle assets of the trust fund with the assets of the Employees Retirement System of Texas, including its trust fund and the law enforcement and custodial officer supplemental retirement fund, for investment purposes, as long as proportionate ownership records are maintained and credited.
(b) The board of trustees shall:
(1) invest the assets of the retirement system as a single fund without distinction as to their source; and
(2) hold securities purchased with the assets collectively for the proportionate benefit of all accounts of the system established under § 840.305(b).

(c) For purposes of the investment authority of the board of trustees under § 67, Article XVI, Texas Constitution, “securities” means any investment instrument within the meaning of the term as defined by § 4001.068, 15 U.S.C. § 77b(a)(1), or 15 U.S.C. § 78c(a)(10).