In a major boost to Sixth Avenue's Midtown corporate corridor, Mitsubishi-owned MUFG Union Bank, N.A., has signed one of the year's largest new leases - 209,700 square feet on three lower floors at Rockefeller Group's 1221 Sixth Ave., aka the McGraw-Hill Building. My colleague Lois Weiss reported in July that MUFG's parent, Bank of Tokyo/Mitsubishi, was negotiating for a somewhat larger block of space on vacant lower floors of 1221 Sixth.