The mechanism of RNA degradation in Escherichia coli involves endonucleolytic 
cleavage, polyadenylation of the cleavage product by poly(A) polymerase, and 
exonucleolytic degradation by the exoribonucleases, polynucleotide phosphorylase 
(PNPase) and RNase II. The poly(A) tails are homogenous, containing only 
adenosines in most of the growth conditions. In the chloroplast, however, the 
same enzyme, PNPase, polyadenylates and degrades the RNA molecule; there is no 
equivalent for the E. coli poly(A) polymerase enzyme. Because cyanobacteria is a 
prokaryote believed to be related to the evolutionary ancestor of the 
chloroplast, we asked whether the molecular mechanism of RNA polyadenylation in 
the Synechocystis PCC6803 cyanobacteria is similar to that in E. coli or the 
chloroplast. We found that RNA polyadenylation in Synechocystis is similar to 
that in the chloroplast but different from E. coli. No poly(A) polymerase enzyme 
exists, and polyadenylation is performed by PNPase, resulting in heterogeneous 
poly(A)-rich tails. These heterogeneous tails were found in the amino acid 
coding region, the 5' and 3' untranslated regions of mRNAs, as well as in rRNA 
and the single intron located at the tRNA(fmet). Furthermore, unlike E. coli, 
the inactivation of PNPase or RNase II genes caused lethality. Together, our 
results show that the RNA polyadenylation and degradation mechanisms in 
cyanobacteria and chloroplast are very similar to each other but different from 
E. coli.