“Public agency” means the state, any city, county, city and county, district, municipal or public corporation or any instrumentality thereof, or boards and committees established under Chapter 1 (commencing with Section 58601) or 2 (commencing with Section 59501) of Part 1 of Division 21 of the Food and Agricultural Code, Chapter 754 of the Statutes of 1933, as amended, or Chapter 307 of the Statutes of 1935, as amended, the employees of which constitute one or more coverage groups or retirement system coverage groups.