The use of mortgage insurance in securitized subprime mortgage security deals picked up steam in late 2000, according to new reports issued by two rating agencies. In its quarterly report on trends in the residential mortgage-backed securities market, Standard & Poor’s said that 45 percent of the subprime transactions it rated during the fourth quarter carried mortgage insurance. That marked a significant increase from the third quarter, when just 12 percent of S&P-rated deals contained