The Role of Exoribonucleases and MutT Pyrophosphohydrolase in the Surveillance of Oxidatively-damaged RNA

Three important exoribonucleases degrading RNAs in sequence-independent
manner, RNase II, RNase Rand polynucleotide phosphorylase (PNPase), were shown to
protect cells against oxidative stress. This is presumably due to the function of the
exoribonucleases in the removal of oxidized RNA in cells. MutT pyrophosphohydrolase
.was previously reported to scavenge oxidized nucleotides 8-oxoGTP and 8-oxoGDP,
prevent their incorporation into RNA. Deficiency of MutT may lead to an increase in the
level of 8-oxoG in RNA, which may enhance the requirement of the RNA surveillance
function of the exoribonucleases. This study focuses on the roles of the RNA-degradation
exoribonucleases in the removal of oxidatively-damaged RNA in the mutT background.
This work shows that mutT mutation enhances the sensitivity of the RNase mutants to
hydrogen peroxide. Growth defect of the pnp mutT mutant was detected even under
normal aeration, but was rescued to the level of pnp mutant under anaerobic conditions. The pnp mutT mutant shows high mutator activity observed from LacZ reporter system
and high level of 8-oxoG in RNA, strongly suggest that PNPase is responsible for
removing 8-oxoG containing RNAs elevated in mutT background. Additionally, genetic
instability observed from the mutant lacking RNase II and MutT supports the idea that
RNase II may adopt a distinct pathway to reduce deleterious effect from oxidation
challenge.