Any city of the first class, city of the second class, or village in the State of Nebraska, which owns and operates public utilities consisting of a waterworks plant, water system, sanitary sewer system, gas plant, gas system, electric light and power plant, or electric distribution system, may pay for extensions and improvements to any of such public utilities by issuing and selling its combined revenue bonds and securing the payment thereof by pledging and hypothecating the revenue and earnings of any two or more of such public utilities and may enter into such contracts in connection therewith as may be necessary or proper. Such combined revenue bonds shall not be general obligations of the city or village issuing the bonds and no taxes shall be levied for their payment but such bonds shall be a lien only upon the revenue and earnings of the public utilities owned and operated by the municipality and which are pledged for their payment.