Any State bank may purchase for its own account marketable investment securities without regard to any other liability to the bank of the issuer, maker, obligor, or guarantor of any marketable investment securities, but the total amount of the marketable investment securities of any one issuer, maker or obligor held by the bank or for its account at any one time shall not exceed 20% of its unimpaired capital and unimpaired surplus. As used in this Section the term “marketable investment securities” means marketable obligations evidencing indebtedness of any person in the form of bonds, notes, or debentures commonly known as investment securities; obligations identified by certificates of participation in investments the bank could have invested in directly; and includes certificates of participation in open end investment companies registered with the Securities and Exchange Commission pursuant to the Investment Company Act of 1940 and Securities Act of 1933 commonly referred to as mutual or money market funds, provided the portfolios of those investment companies consist of investments that a bank could invest in directly. Marketable investment securities shall be rated in the top 4 rating categories by national rating services and designated as “investment grade” or “bank quality investments” securities. The rating restriction on marketable investment securities does not apply to securities that are issued by a public agency as defined in Section 1 of the Public Funds Investment Act.