Abstract
The transcriptional organization of the plastid psbK- psbI- psbD- psbC gene cluster in rice seedlings was investigated. In total, seven classes of overlapping transcripts (RNAs a-g) based on the 5′ ends were deduced and mapped in detail. Light induced the accumulation of class d RNAs, which were not detected in the dark-grown plants, and enhanced the accumulation of all other transcripts. An in vitro transcription assay demonstrated that there are four promoters residing in the gene cluster, which drive four overlapping transcription units to produce RNAs of classes a, c, d and f. An in vitro capping assay identified these transcription initiation sites, which coincide with the mapped 5′ termini of the respective RNAs. The heterogenous 5′ termini of class d RNAs were proved to arise from transcriptional initiation mediated by the light-responsive promoter residing between psbI and psbD. It is suggested that RNAs of class b, e and g are processed from precursor transcripts by some plastid nucleolytic activities.
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