Most plastidial tRNAs are transcribed by the PEP from upstream c70-type promoters. However, transcription from internal promoters is assumed for some tRNA genes such as the spinach trnS, trnR, and trnT (Gruissem et al. 1986; Cheng et al. 1997b) as well as trnS, trnH, and trnR from mustard (Neuhaus and Link 1990; Nickelsen and Link 1990; Liere and Link 1994), and the Chlamydomonas trnE gene (Jahn 1992). Transcription of the spinach trnS gene is initiated twelve nucleotides upstream of the mature tRNA coding region (Wu et al. 1997). In vitro assays demonstrated that the coding region (+1/+93) promoted basal levels (8%) of transcription. Inclusion of an AT-rich sequence stretch between -31 and -11 upstream of the coding region restored wild type promoter strength. However, no sequences resembling either NEP or PEP promoters were found in this region. As most tRNAs, the trnS coding region contains sequences resembling the A and B blocks of nuclear tRNA promoters transcribed by the eukaryotic RNA polymerase III (Galli et al. 1981; Geiduschek et al. 1995). The tRNAArg(ACG) gene from Pelargonium zonale was efficiently transcribed in Xenopus oocyte nuclei (Hellmund et al. 1984), suggesting that the plastidial tRNAs may be transcribed by an RNA polymerase III-type enzyme. The biochemical properties and enzyme composition of such a transcription activity, however, remain to be determined. Thus far, in silico analyses of the Arabidopsis genome did not reveal a plastid-targeted poly-merase of this type (Liere and Börner, unpublished). Alternatively, such tRNAs may be transcribed by specialized NEP or PEP enzymes associated with distinct transcription factors recognizing internal promoter structures.
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