Chloroplast PNPase exists as a homo-multimer enzyme complex that is distinct from the Escherichia coli degradosome

In Escherichia coli, the exoribonuclease polynucleotide phosphorylase (PNPa se), the endoribonuclease RNase E, a DEAD-RNA helicase and the glycolytic e nzyme enolase are associated with a high molecular weight complex, the: deg radosome. This complex has an important role in processing and degradation of RNA. Chloroplasts contain an exoribonuclease homologous to E. coli PNPas e. Size exclusion chromatography revealed that chloroplast PNPase elutes as a 580-600 kDa complex, suggesting that it can form an enzyme complex simil ar to the E. coli degradosome. Biochemical and mass-spectrometric analysis showed, however, that PNPase is the only protein associated with the 580-60 0 kDa complex. Similarly, a purified recombinant chloroplast PNPase also el uted as a 580-600 kDa complex after gel filtration chromatography. These re sults suggest that chloroplast PNPase exists as a homo-multimer complex. No other chloroplast proteins were found to associate with chloroplast PNPase during affinity chromatography. Database analysis of proteins homologous t o E. coli RNase E revealed that chloroplast and cyanobacterial proteins lac k the C-terminal domain of the E. coli protein that is involved in assembly of the degradosome. Together, our results suggest that PNPase does not for m a degradosome-like complex in the chloroplast. Thus, RNA processing and d egradation in this organelle differ in several respects from those in E. co li.