Polynucleotide Phosphorylase Functions as Both an Exonuclease and a Poly(A) Polymerase in Spinach Chloroplasts

Abstract

The molecular mechanism of mRNA degradation in the chloroplast consists of sequential events including endonucleolytic cleavage, the addition of poly(A)-rich sequences to the endonucleolytic cleavage products, and exonucleolytic degradation by polynucleotide phosphorylase (PNPase). In Escherichia coli,polyadenylation is performed mainly by poly(A)-polymerase (PAP) I or by PNPase in its absence. While trying to purify the chloroplast PAP by following in vitro polyadenylation activity, it was found to copurify with PNPase and indeed could not be separated from it. Purified PNPase was able to polyadenylate RNA molecules with an activity similar to that of lysed chloroplasts. Both activities use ADP much more effectively than ATP and are inhibited by stem-loop structures. The activity of PNPase was directed to RNA degradation or polymerization by manipulating physiologically relevant concentrations of P_iand ADP. As expected of a phosphorylase, P_i enhanced degradation, whereas ADP inhibited degradation and enhanced polymerization. In addition, searching the completeArabidopsis genome revealed several putative PAPs, none of which were preceded by a typical chloroplast transit peptide. These results suggest that there is no enzyme similar to E. coli PAP I in spinach chloroplasts and that polyadenylation and exonucleolytic degradation of RNA in spinach chloroplasts are performed by one enzyme, PNPase.

ACKNOWLEDGMENTS

We thank A. J. Carpousis for the PAP I antibodies and David Stern, Takahiro Nakamura, Ruth Rott, and Varda Liveanu for providing helpful comments on the manuscript. Special thanks to David Stern for help in editing the manuscript.

This work was supported by grants from the Israel Science Foundation, the Israel-Japan Corporation Foundation, and the Israel-USA Binational Agriculture Research and Development Foundation (BARD).