In an effort to increase the thermostability of Moloney Murine Leukemia Virus reverse transcriptase (MMLV RT), we screened random and site-saturation libraries for variants that show increased resistance to thermal inactivation. We discovered five mutations E69K, E302R, W313F, L435G and N454K that collectively increase the half-life of MMLV RT at 55 degrees C from less than 5 min to approximately 30 min in the presence of template-primer. In addition, these mutations alter the thermal profile by increasing specific activity of the pentuple mutant (M5) over a broad range of cDNA synthesis temperatures (25-70 degrees C). We further show that M5 generates higher cDNA yields and exhibits better RT-PCR performance compared to wild-type RT when used at high temperature to amplify RNA targets containing secondary structure. Finally, we demonstrate that M5 exhibits tighter binding (lower K(m)) to template-primer, which likely protects against heat inactivation.