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DEVELOPMENT AND HORMONE ACTION

AtSig5 Is an Essential Nucleus-Encoded Arabidopsis -Like Factor¹

Department of Biochemistry, University of Nebraska-Lincoln, N250 Beadle Center, Lincoln, Nebraska 68588–0664

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.

¹ This work was supported by the National Research Initiative Competitive Grants Program/U.S. Department of Agriculture (grant nos. 97–35301–4514 and 00–35301–9016 to L.A.A.).

^* Corresponding author; e-mail lallison{at}creighton.edu; fax 402–472–7842.

Received November 15, 2002; returned for revision January 13, 2003; accepted February 23, 2003.

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