The present invention provides a method for obtaining thermostable enzymes. The present invention also provides variants of DNA polymerase I from Thermus aquaticus. The present invention further provides methods of identifying mutant DNA polymerases having enhanced catalytic activity. The present invention also provides polynucleotides, expression systems, and host cells encoding the mutant DNA polymerases. Still further, the present invention provides a method to carry out reverse transcriptase-polymerase chain reaction (RT-PCR) and kits to facilitate the same.