Chloroplast development affects expression of phage‐type RNA polymerases in barley leaves

Abstract

SummaryWe have identi®ed the barley gene and cDNA encoding the plastid phage-type RNA polymerase (RNAP), nuclear-encoded plastid RNAP (RpoTp), and the nearly full-length cDNA of the mitochondrial RNAP, nuclear-encoded mitochondrial RNAP (RpoTm). RpoTp spans more than 9000 nt, consists of 19 exons and 18 introns, gives rise to a 3632-nt mRNA and is localized to the long arm of chromosome 1 (7H). The length of the deduced polypeptide is 948 residues. The mRNA levels of RpoTp and RpoTm were determined in roots and primary leaf sections of 7-day-old barley seedlings of the albostrians mutant, which were either phenotypically normal and exhibited a gradient of chloroplast development, or contained ribosome-de®-cient undifferentiated plastids. Transcript levels of RpoTp and RpoTm in almost all sections reached higher concentrations in plastid ribosome-de®cient leaves than in the wild-type material, except in the most basal part of the leaf. These data indicate a role of plastid-to-nucleus signalling in the expression of the two RpoT genes. The mRNA levels of the plastid genes, b-subunit of plastid-encoded RNAP (rpoB), proteolytic subunit of the Clp protease (clpP) and ribosomal protein Rpl2 (rpl2), all known to be transcribed by the nuclearencoded RNAP (NEP), followed closely the pattern of RpoTp mRNA accumulation, strongly suggesting that RpoTp and NEP are identical. Transcripts of RpoTm and RpoTm-transcribed mitochondrial genes cytochrome oxidase subunit 2 (coxII) and ATPase subunit 9 (atp9) accumulated to the highest levels in the most basal parts of the leaf and declined considerably towards the leaf tip with a pronounced reduction in green versus white leaves. Our data revealed a marked in¯uence of the developmental stage of the plastid on the expression and activity of organellar phage-type RNAPs and their target genes. Thus, interorganellar cross-talk in the regulated expression of nuclear-encoded plastid and mitochondrial RNAP genes might be a key element governing the concerted expression of genes located within plastids, mitochondria and the nucleus of the plant cell.

Effects of Reduced Chloroplast Gene Copy Number on Chloroplast Gene Expression in Maize

“…The overrepresentation of NEP transcripts in w2 mutants is consistent with reports that NEP-mediated transcription increases in response to the loss of plastid ribosomes (Emanuel et al, 2004) or mutations in PEP genes (Legen et al, 2002). However, the very different degrees to which different NEP transcripts are overrepresented in response to the loss of plastid DNA indicates that differential RNA stability or transcription contribute to this phenomenon.…”

“…These genes are transcribed primarily by NEP, whose mRNA increases 8-fold in barley (Hordeum vulgare) mutants lacking PEP due to a plastid ribosome deficiency (Emanuel et al, 2004). However, the particularly dramatic overrepresentation of rpoB mRNA when plastid DNA content is reduced suggests the existence of a dedicated regulatory mechanism affecting the synthesis or stability of this transcript, which encodes catalytic subunits of the PEP enzyme.…”

Arabidopsis Phage-Type RNA Polymerases: Accurate in Vitro Transcription of Organellar Genes

“…Similarly, RpoTmp synthesized exclusively nonspecific RNA products from plastid DNA templates regardless of which class of NEP promoter was present on the template. The lack of promoter specificity of RpoTmp is particularly remarkable, as the lineage leading to RpoTmp has evolutionarily diverged from RpoTm more recently than RpoTp that still possesses this feature (Emanuel et al, 2004;Azevedo et al, 2006). RpoTp orthologs have been identified in monocotyledonous and dicotyledonous angiosperms (Weihe, 2004) but appear to be absent in the green alga Chlamydomonas reinhardtii (A. Weihe, unpublished data) and in the moss Physcomitrella (Kabeya et al, 2002;Richter et al, 2002).…”

A Functional Component of the Transcriptionally Active Chromosome Complex, Arabidopsis pTAC14, Interacts with pTAC12/HEMERA and Regulates Plastid Gene Expression

“…It contains more than 120 genes important for its development and function (Sugita and Sugiura, 1996;Sato et al, 1999). The transcriptional machinery plays an important role in the regulation of chloroplast development at different developmental stages (Mullet, 1993;Emanuel et al, 2004;Zoschke et al, 2007). Higher plant cells possess at least two types of RNA polymerases: a plastid-encoded bacterial-type RNA polymerase (PEP), which is a multisubunit eubacterium-type enzyme, and a nucleusencoded phage-type RNA polymerase (NEP) that resembles the single-subunit type shared with phage T3/T7 and mitochondrial enzymes (Hu and Bogorad, 1990;Igloi and Kö ssel, 1992;Lerbs-Mache, 1993;Pfannschmidt and Link, 1997;Liere and Maliga, 2001).…”