* Firms with large exposure to China underperform
* Global trade tensions, U.S. foreign policy unnerve market
* Nikkei dropped 4.9 percent for the week
* Topix posts biggest turnover since mid-February
By Ayai Tomisawa
TOKYO, March 23 (Reuters) - Japan's Nikkei share average tumbled on Friday to its lowest level in more than five months as concerns over escalating global trade tensions triggered a spike in the yen, with machinery makers hit particularly hard.
U.S. President Donald Trump signed a presidential memorandum on Thursday that will target up to $60 billion in Chinese goods with tariffs. They will kick in after a 30-day consultation period.
Japan's benchmark Nikkei ended 4.5 percent lower at 20,617.86, its biggest daily percentage drop since early February and its lowest closing level since Oct. 3.
For the week, the benchmark index tumbled 4.9 percent, the biggest weekly percentage fall since early February as well.
The broader Topix declined 3.6 percent to 1,664.94, with all of its 33 subsectors in negative territory. Trade was heavy, posting a turnover of 3.6 trillion yen, the biggest since mid-February.
"When there is a trade war about to start between the world's two largest economies, who is not worried about the impact on the real economy?," asked Norihiro Fujito, a senior investment strategist at Mitsubishi UFJ Morgan Stanley Securities.
Japanese companies with heavy exposure to China fell sharply, with the machinery sector slumping 5.6 percent, the biggest loser on the board. Construction equipment maker Komatsu tumbled 6.3 percent.
Machine tool makers also were hammered, with Makino Milling Machine sliding 6.1 percent and Okuma Corp nosediving 7.4 percent.
Industrial robot maker Fanuc Corp fell 4.4 percent, semiconductor equipment makers Advantest and Tokyo Electron dropped 4.1 percent and 5.7 percent, respectively.
News that Trump replaced H.R. McMaster as national security adviser with John Bolton, a hawk who has advocated using military force against North Korea and Iran, also hurt investor sentiment.
"Uncertainty over foreign policy of the Trump administration is increasing and it may last for a while," said Masahiro Yamaguchi, a senior market analyst at SMBC Trust Bank.
The dollar fell as low as 104.635 yen in early Asian trade on Friday, its lowest level since November 2016, stoking concerns that the Japanese currency's strength will sap the earnings of Japanese companies.
Exporters also faced heavy selling, with Honda Motor Corp falling 5.3 percent and TDK Corp shedding 5.2 percent.
Reporting by Ayai Tomisawa; Editing by Kim Coghill