Who can initiate an optional referendum at the national level?

TheConstitution of the Republic of Turkey

(Adopted in7 November 1982, last amended 17 October 2001)

http://www.anayasa.gov.tr/images/loaded/pdf_dosyalari/THE_CONSTITUTION_OF_THE_REPUBLIC_OF_TURKEY.pdf

Part 7, I,Amendment of the Constitution, Participation in Elections and Referenda,Article 175. (Amended on 17 May 1987)

[…]The President of the Republic mayrefer the laws related to the Constitutional amendments for furtherconsideration. If the Assembly adopts the draft law referred by the Presidentby a two-thirds majority, the President may submit the law to referendum. […]

[…]A law on the Constitutionalamendment adopted by a two- thirds majority of the total number of members ofthe Turkish Grand National Assembly directly or if referred back by thePresident for further consideration, or its articles as considered necessarymay be submitted to a referendum by the President. […]

[…]The Turkish Grand NationalAssembly, in adopting the laws related to the Constitutional amendment, shallalso decide on which provisions shall be submitted to referendum together andwhich shall be submitted individually. […]