In the absence of any express provisions to the contrary, whenever any general statute, special act, regulation, trust indenture, will, or other instrument governing the investment powers of trustees directs, requires, authorizes, or permits investment in United States government obligations, a trustee may invest in and hold these obligations, either directly or in the form of interests in an investment company or investment trust registered under the Investment Company Act of 1940, 15 U.S.C. § 80a-1 et seq., the portfolio of which is limited to United States government obligations and to repurchase agreements fully collateralized by these obligations, and that is rated in the top rating category of any nationally recognized rating services.