Transcription of chloroplast genes is subject to control by nucleus-encoded proteins. The chloroplast-encoded RNA polymerase (PEP) is a eubacterial-type RNA polymerase that is presumed to assemble with nucleus-encoded -factors mediating promoter recognition. Recently, families of -factor genes have been identified in several plants including Arabidopsis. One of these genes, Arabidopsis SIG5, encodes a -factor, AtSig5, which is phylogenetically distinct from the other family members. To investigate the role of this plant -factor, two different insertional alleles of the SIG5 gene were identified and characterized. Heterozygous mutant plants showed no visible leaf phenotype, but exhibited siliques containing aborted embryos and unfertilized ovules. Our inability to recover plants homozygous for a SIG5 gene disruption indicates that SIG5 is an essential gene. SIG5 transcripts accumulate in flower tissues, consistent with a role for AtSig5 protein in reproduction. Therefore, SIG5 encodes an essential member of the Arabidopsis -factor family that plays a role in plant reproduction in addition to its previously proposed role in leaf chloroplast gene expression.Transcription of plant mitochondrial and plastid genomes relies on nucleus-encoded RNA polymerases resembling those of the T3 and T7 bacteriophage, as well as promoter selectivity factors for these enzymes (for review, see Hess and Bö rner, 1999;Liere and Maliga, 2001). In addition, plastids, but not mitochondria, require an organelle-encoded RNA polymerase (PEP) for transcription of many genes on the organelle genome (Allison and Maliga, 1996;Hajdukiewicz et al., 1997;Serino and Maliga, 1998). PEP is similar in structure to Escherichia coli RNA polymerase and, like the eubacterial enzyme, is thought to assemble with -factors to achieve promoter-specific transcription initiation. Although the subunits of the PEP catalytic core are plastidencoded, the putative promoter-specificity -factors for PEP are encoded in the plant nucleus. Therefore, even the organelle-encoded transcription machinery is subject to nuclear control.Recently, -factor gene families containing as many as six members have been identified in several plant species including Arabidopsis and maize (Zea mays; for review, see Allison, 2000). Within a species, the sigma-like proteins share approximately 35% overall amino acid sequence identity and contain the conserved domains found in the principal -factors of all eubacteria (Helmann and Chamberlin, 1988). Genetic analyses of the eubacterial -factors, as well as more recent structural studies, have supported the importance of these conserved domains in such eubacterial transcription functions as promoter recognition, interaction with the RNA polymerase core enzyme, and transcription initiation (Gross et al., 1998;Burgess and Anthony, 2001;Campbell et al., 2002;Murakami et al., 2002). The conservation of primary structure among plant and eubacterial -factors is sufficiently high to permit recombinant plant -factors to function with E. coli core RNA po...

SummaryPlants contain nuclear gene families that encode proteins related to the principal sigma factors of eubacteria. As sigma factors function in transcription, the plant proteins have been presumed or demonstrated to associate with the eubacteria-like RNA polymerase of chloroplasts. In maize, ®ve sig cDNA sequences have been reported, and four of the products are present in plastids as predicted. However, in vitro chloroplast import assays and computer algorithms gave ambiguous results with the ®fth protein, ZmSig2B. Unlike the other maize sigma factors, ZmSig2B is expressed throughout developing seedling leaves, as well as in roots and etiolated tissues. To determine the subcellular location of ZmSig2B, we have now used immunoblot assays to show that it co-puri®es with both mitochondria and plastids. Its NH 2 -terminal 153 amino acids, translationally fused to green¯uorescent protein (GFP), targeted GFP to chloroplasts and mitochondria in bombarded maize leaves. A putative role for ZmSig2B in mitochondrial transcription is supported by its presence in a maize mitochondrial transcription extract. ZmSig2B also exhibits the expected properties of a chloroplast sigma factor: recombinant ZmSig2B binds to a chloroplast promoter and initiates transcription in vitro when combined with Escherichia coli core RNA polymerase. Therefore ZmSig2B is an unusual nucleus-encoded sigma factor that appears to function in both chloroplasts and mitochondria.