Differential Regulation of Nuclear Genes during Blue Light‐dependent Chloroplast Differentiation in Cultured Plant Cells

Nuclei capable of RNA synthesis were isolated from both dark-grown and blue light-irradiated cell cultures of Chenopodium rubrum. Analysis of the heterodisperse RNA sequences synthesized showed that the SSU and LHCP gene as well as early light-induced genes were specifically transcribed. The relative transcription rate of their mRNAs increased with the onset of blue light irradiation reaching a maximum at about 12 h, then dropped to the dark level. A comparison with the steady-state concentration of the same mRNAs in vivo revealed a substantial difference between the two groups of genes: For the three early light-induced genes the in vitro transcription rate is in good accordance with the observed steady-state level of their transcripts supporting the notion that the blue light-dependent rapid and transient accumulation in vivo is caused by a temporary increase in the transcription rate of the corresponding genes. In contrast, for the SSU and LHCP gene, the changes in transcription rate do not follow those in steady-state concentration; thus the observed steady increase of the latter in blue light cannot be explained as an enhancement in the transcription rates of both genes, but is a likely consequence of post-transcriptional events.