After 43 years in Mission Valley, the San Diego Union-Tribune opened its new downtown offices May 16. About 275 employees vacated the former location – near the Interstate 8-state Route 163 interchange – late last week. The daily newspaper’s new home, at 600 B St., occupies four floors, encompassing around 60,000 square feet, of a 24-story building. A $40 million, 15-year lease was signed. The paper was located at Second Avenue and Broadway until 1973, when it moved to the highly visible Mission Valley building. The move comes about a month after the local NBC TV outlet left downtown and returned to Kearny Mesa after an absence of about 15 years.