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Identification and characterization of a family of sigma factors from Arabidopsis thaliana
Plant sigma-like factors have long been suggested to be active components of the plastid-encoded RNA polymerase in transcribing the plastid genome. Since no sigma-like sequences were found in the plastid genome, it was suggested that they are encoded in the nucleus, translated in the cytoplasm and targeted to the plastid. Five different sigma factor genes from Arabidopsis, SIG1 through SIG5, were identified from the Arabidopsis genome by a computer based strategy and cloned. These genes share high sequence similarity with bacterial sigma factors, suggesting that these plant proteins may function in a similar fashion. Transcript levels for all five SIG genes were regulated similarly: RNA accumulated at higher levels in leaves than in roots, and in leaves when induced by light treatment. Several reverse genetics approaches were used to dissect individual sigma factor function. Antisense SIG2 transgenic plants were recovered with dramatically reduced levels of AtSig2 protein and no visible leaf phenotype. Northern blot analysis indicated a potential role for AtSig2 in regulating photosystem gene expression. ^ The SIG5 gene is unique because of its phylogenetic distance from other sigma family members. SIG5 transcripts from flower tissue contain a longer splice variant that is not present in leaves. The protein product of this splice variant is capable of targeting a fused reporter protein to both chloroplasts and mitochondria, while the protein product of the leaf transcript only exhibited targeting to chloroplasts. Two Arabidopsis mutants harboring T-DNA insertion null alleles of SIG5 by were recovered from two different mutant populations. These mutants, sig5-1 and sig5-2, exhibited a seed phenotype, which resulted in our inability to recover homozygous sig5/sig5 plants, indicating that this plant sigma factor is an essential gene. ^ These data suggest that regulation and function of plant sigma factor family is diverse. SIG2 might be functionally redundant with other sigma factors, while SIG5 is essential for plant reproduction. Dual-targeting of SIG5 may indicate a novel function for this sigma factor protein. ^
Biology, Molecular|Chemistry, Biochemistry
Yao, Junlan, "Identification and characterization of a family of sigma factors from Arabidopsis thaliana" (2002). ETD collection for University of Nebraska - Lincoln. AAI3074114.