In the event any trustee, provided for in subdivisions (d) and (e) of section five of this article leaves the employ of a participating public employer, or fails to attend three consecutive meetings of the board of trustees, unless in each case excused for cause by the remaining trustees attending such meeting or meetings, he shall be considered to have resigned from the board and the board shall, by resolution, declare his office of trustee vacated. If a vacancy occurs in the office of such trustee, the Governor shall, within thirty days from and after the date of the vacancy, fill the vacancy, by appointment, for the unexpired term.