A corporation doing a trust business may invest trust funds in savings accounts with itself to the extent that deposits are insured by an agency of the federal government. Additional trust funds may be so invested by the corporation only if it first sets aside under the control of its trust department as collateral security:
(1) Direct obligations of the United States or other obligations fully guaranteed by the United States as to principal and interest; or
(2) Bonds or other obligations which constitute general obligations of any state of the United States or municipal subdivision thereof.
The securities so deposited or securities substituted therefor as collateral shall at all times be at least equal in market value to the amount of the funds so deposited.