chloroplast transcription, transcription regulation; tRNAGlu, plastid tRNA, mitochondrial transcription, Arabidopsis
In Arabidopsis and other eudicots, transcription of chloroplast genes is performed by three RNA polymerases: one bacterial-type, plastid-encoded RNA polymerase (PEP) and two nuclear-encoded phage-type RNA polymerases (NEP = RpoTp and RpoTmp). Previously published data suggested that a switch from NEP to PEP activity occurs during early leaf development due to inactivation of RpoTp by binding of chloroplast tRNAGlu. We have reinvestigated the postulated inhibitory activity of tRNAGlu using an in vitro transcription assay and by studying the binding of plastidial tRNAGlu, tRNAVal, and tRNAGly to RpoTp, RpoTmp and the mitochondrial RNA poly-merase RpoTm. We observed an inhibition of RpoTp activity by adding tRNAGlu to the in vitro transcription assay. Although these results are in agreement with the proposed function of tRNAGlu as a regulator of RpoTp activity, the following observations made during the current study suggest that tRNAGlu does not play a role in specifically regulating NEP activity: (i) the inhibitory effect of tRNAGlu on RpoTp activity required a rather high molar ratio of tRNA and protein (ii) not only tRNAGlu but also the two other plastid tRNAs studied (tRNAVal, tRNAGly) were found to bind to RpoTp, and (iii) the binding of tRNAs was not specific for RpoTp but was observed for all three Arabidopsis phage-type RNA poly-merases.