Comparative analysis of plastid gene expression in tobacco chloroplasts and proplastids: Relationship between transcription and transcript accumulation

Abstract

We compared the modes of plastid gene expression between chloroplasts and proplastids in tobacco (Nicotiana tabacum L.). Chloroplast-nuclei (chloroplast-nucleoids) and proplastid-nuclei (proplastid-nucleoids) were isolated from mature leaves and cultured cells (line BY-2), respectively, and their transcriptional activities in vitro were compared. Overall transcriptional activity of the isolated chloroplast-nuclei (250 pmol UTP (μg DNA)-1 h-1) was approximately 25 times that of the isolated proplastid-nuclei (9.7 pmol UTP (μg DNA)-1 h-1). This difference in overall transcriptional activities was accompanied by intensive modulation of the relative transcriptional activities of individual genes. Differences in the transcriptional activities of nine plastid genes (psbA, atpA, rpoB, psaA/B, atpB, rbcL, petB, rpl16, and rrn23) between the isolated chloroplast- and proplastid-nuclei correlated moderately with differences in the amounts of corresponding transcripts accumulating in leaves and cultured cells. This suggests that transcriptional regulation is responsible, to a considerable extent, for the differential accumulation of plastid transcripts. Possible mechanisms underlying the differential transcription in proplastids and chloroplasts are discussed in relation to the existence of multiple RNA polymerases and structural changes involving plastid-nuclei.