Chloroplast rps15 and the rpoB/C1/C2 gene cluster are strongly transcribed in ribosome-deficient plastids: evidence for a functioning non-chloroplast-encoded RNA polymerase.

Abstract

Transcription of plastid genes and transcript accumulation were investigated in white leaves of the albostrians mutant of barley (Hordeum vulgare) and in heat‐bleached leaves of rye (Secale cereale) as well as in normal green leaves of both species. Cells of white leaves of the mutant and cells of heat‐bleached leaves bear undifferentiated plastids lacking ribosomes and, consequently, plastid translation products, among them the subunits of a putative chloroplast RNA polymerase encoded by the plastid genes rpoA, B, C1 and C2. The following results were obtained. (i) Plastid genes are transcribed despite the lack of chloroplast gene‐encoded RNA polymerase subunits. The plastid origin of these transcripts was proven. This finding provides evidence for the existence of a plastid RNA polymerase encoded entirely by nuclear genes. (ii) Transcripts of the rpo genes and of rps15, but not of genes involved in photosynthesis and related processes (psbA, rbcL, atpI‐H), were abundantly accumulated in ribosome‐deficient plastids. In contrast, chloroplasts accumulated transcripts of photosynthetic, but not of the rpo genes. (iii) Differences in transcript accumulation between chloroplasts and ribosome‐deficient plastids are due to different relative transcription rates and different transcript stability. (iv) The observed differences in transcription are not caused by an altered pattern of methylation of plastid DNA. Thus, the prokaryotic plastid genome of higher plants is transcribed by two RNA polymerases. The observed differences in transcription between chloroplasts and undifferentiated plastids might reflect different functions of the two enzymes.

Involvement of Protein Kinase and Extraplastidic Serine/Threonine Protein Phosphatases in Signaling Pathways Regulating Plastid Transcription and the psbD Blue Light- Responsive Promoter in Barley

“…It is tempting to speculate that the transcription activity that was insensitive to inhibition by OA was due to a second plastid RNA polymerase that was not regulated by PP1 and PP2A. A nuclear-encoded, DNAdependent RNA polymerase exhibits different mechanisms of control, developmental regulation, and subunit composition than the plastid-encoded RNA polymerase (Hess et al, 1993;for review, see Mullet, 1993). Having unique signaling pathways that regulate separate plastid RNA polymerases would allow for differential biogenesis and expression of the plastid transcription apparatus.…”

Effects of chloroplast dysfunction in a subpopulation of leaf mesophyll cells on photosynthetic and respiratory activities of a whole leaf: A study using variegated leaves of Hedera helix L.

“…The various causes of this variegation include plastid ribosome deficiency (Han et al 1992, Hess et al 1993, alteration of plastid genome (Stoike and Sears 1998), mitochondrial dysfunction (Martínez-Zapater et al 1992, Sakamoto et al 1996, and deficiencies in carotenoid biosynthesis (Wetzel et al 1994). Variegated plants have played prominent roles in research on genetics (e.g., discovery of non-Mendelian inheritance) and morphology (e.g., analysis of cell lineage in leaf development).…”

A tRNA gene transcription initiation site is similar to mRNA and rRNA promoters in plant mitochondria

“…In plastids rRNA genes are duplicated and evidence for an additional RNA-polymerase (and/or its respective cofactors) has been described, which may provide a differential tanscriptional activity (3)(4)(5). Only few of the plastid tRNA genes are located in the inverted repeat region, while most of the others appear to be cotranscribed with protein coding genes (6)(7)(8).…”