Section 5. An initiative petition shall be submitted not less than one hundred twenty days before the statewide election at which the measure is to be voted upon. A referendum petition may be submitted only within ninety days after the filing of the measure with the secretary of state. The submission of a petition shall suspend the operation of any measure enacted by the legislative assembly except emergency measures and appropriation measures for the support and maintenance of state departments and institutions. The submission of a petition against one or more items or parts of any measure shall not prevent the remainder from going into effect. A referred measure may be voted upon at a statewide election or at a special election called by the governor.