Reverse Transcription Of Retroviruses

The retroviral reverse transcriptases (RT) possess basically of two major enzymatic activities: a DNA polymerase capable of copying either RNA or DNA template, and a ribonuclease H (RNase H) active on the RNA-DNA hybrids. Interestingly, a special oligoribonucleotide species which contained the 5'-terminus of RNA template, was generated by the RT-associated RNase H activity, when the RT reached and halted at the end of an RNA template. This residual 5'-terminal fragment was about 14-18 nt in length depending upon the reaction conditions (i.e. the sources of RTs and templates, etc.), and over 80% of this species was still base-pair to its complementary DNA products. These data indicate that the cleavage site for the RNase H domain of RT is probably held at about 14-18 nt behind the polymerization site for the DNA polymerase domain. These two domains are not functionally independent. Furthermore, under a certain experimental condition such an oligoribonucleotide could also act as a primer for the synthesis of a second-strand DNA. Even though this process might be suppressed in vivo, it still...