It shall be the duty of the secretary and treasurer of said board of regents to forward to the bank at which said bonds ar [are] payable, prior to the date on which any coupons or any principal amount of any of said bonds shall mature, out of the “interest and retirement fund” a sufficient sum of money to meet said coupons and maturing bonds as the same become due, plus any service charge which said bank shall be entitled to receive for its services.