The Tf1 retrotransposon represents a group of long terminal repeat retroelements 
that use an RNA self-primer for initiating reverse transcription while 
synthesizing the minus-sense DNA strand. Tf1 reverse transcriptase (RT) was 
found earlier to generate the self-primer in vitro. Here, we show that this RT 
can remove from the synthesized cDNA the entire self-primer as well as the 
complete polypurine tract (PPT) sequence (serving as a second primer for cDNA 
synthesis). However, these primer removals, mediated by the RNase H activity of 
Tf1 RT, are quite inefficient. Interestingly, the integrase of Tf1 stimulated 
the specific Tf1 RT-directed cleavage of both the self-primer and PPT, although 
there was no general enhancement of the RT's RNase H activity (and the integrase 
by itself is devoid of any primer cleavage). The RTs of two prototype 
retroviruses, murine leukemia virus and human immunodeficiency virus, showed 
only a partial and nonspecific cleavage of both Tf1-associated primers with no 
stimulation by Tf1 integrase. Mutagenesis of Tf1 integrase revealed that the 
complete Tf1 integrase protein (excluding its chromodomain) is required for 
stimulating the Tf1 RT primer removal activity. Nonetheless, a double mutant 
integrase that has lost its integration functions can still stimulate the RT's 
activity, though heat-inactivated integrase cannot enhance primer removals. 
These findings suggest that the enzymatic activity of Tf1 integrase is not 
essential for stimulating the RT-mediated primer removal, while the proper 
folding of this protein is obligatory for this function. These results highlight 
possible new functions of Tf1 integrase in the retrotransposon's reverse 
transcription process.