External guide sequences (EGSs) are small RNA molecules
which consist of a sequence complementary to a target mRNA
and render the target RNA susceptible to degradation by
ribonuclease P (RNase P). EGSs were designed to target
the mRNA encoding thymidine kinase (TK) of herpes simplex
virus 1 for degradation. These EGSs were shown to be able
to direct human RNase P to cleave the TK mRNA sequence
efficiently in vitro. A reduction of about 80% in the expression
level of both TK mRNA and protein was observed in human
cells that steadily expressed an EGS, but not in cells
that either did not express the EGS or produced a “disabled”
EGS which carried a single nucleotide mutation that precluded
RNase P recognition. Thus, EGSs may represent novel gene-targeting
agents for inhibition of gene expression and antiviral
activity.