Polynucleotide phosphorylase, RNase II and RNase E play different roles inthe in vivo modulation of polyadenylation in Escherichia coli

Poly(A) tails in Escherichia coil are hypothesized to provide unstructured single-stranded substrates that facilitate the degradation of mRNAs by ribo nucleases. Here, we have investigated the role that such nucleases play in modulating polyadenylation in vivo by measuring total poly(A) levels, polya denylation of specific transcripts, growth rates and cell viabilities in st rains containing various amounts of poly(A) polymerase I (PAP I), polynucle otide phosphorylase (PNPase), RNase II and RNase E. The results demonstrate that both PNPase and RNase II are directly involved in regulating total in vivo poly(A) levels. RNase II is primarily responsible for degrading poly( A) tails associated with 23S rRNA, whereas PNPase is more effective in modu lating the polyadenylation of the Ipp and 16S rRNA transcripts. In contrast , RNase E appears to affect poly(A) levels indirectly through the generatio n of new 3' termini that serve as substrates for PAP I. In addition, wherea s excess PNPase suppresses polyadenylation by more than 70%, the toxicity a ssociated with increased poly(A) levels is not reduced. Conversely, toxicit y is significantly reduced in the presence of excess RNase II. Overproducti on of RNase E leads to increased polyadenylation and no reduction in toxici ty.