The Nikkei stock index closed above the 40,000 threshold for the first time in three months, as exporters were boosted by the weaker yen hitting a fresh 37-year low in the upper 161 range versus the U.S. dollar. The 225-issue Nikkei Stock Average ended up 443.63 points, or 1.12 percent, from Monday at 40,074.69, its highest close since March 29. The broader Topix index finished 32.34 points, or 1.15 percent, higher at 2,856.62, its highest close since Jan. 4, 1990. On the top-tier Prime Market, precision instrument, transportation equipment and insurance were among gainers.