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Molecular characterization of a positively photoregulated nuclear gene for a chloroplast RNA polymerase sigma factor in Cyanidium caldarium

Liu, B., Troxler, R.F.
Proceedings of the National Academy of Sciences of the United States of America 1996 v.93 no.8 pp. 3313-3318
Rhodophycota, structural genes, DNA-binding proteins, DNA-directed RNA polymerase, chloroplasts, nucleotide sequences, amino acid sequences, gene expression, messenger RNA, light, antibodies, precipitin tests, genome, nuclear genome
We have cloned the gene for a putative chloroplast RNA polymerase sigma factor from the unicellular rhodophyte Cyanidium caldarium. This gene contains an open reading frame encoding a protein of 609 amino acids with domains highly homologous to all four conserved regions found in bacterial and cyanobacterial sigma 70-type subunits. When Southern blots of genomic DNA were hybridized to the "rpoD box" oligonucleotide probe, up to six hybridizing bands were observed. Transcripts of the sigma factor gene were undetectable in RNA from dark-grown cells but were abundant in the poly(A)+ fraction of RNA from illuminated cells. The sigma factor gene was expressed in Escherichia coli, and antibodies against the expressed sigma factor fusion protein cross-reacted with a 55-kDa protein in partially purified chloroplast RNA polymerase. Antibodies directed against a cyanobacterial RNA polymerase sigma factor also cross-reacted with a 55-kDa protein in the same enzyme preparation. Immunoprecipitation experiments showed that this enzyme preparation contains proteins with the same molecular weights as the alpha, beta, beta', and ,beta" subunits of chloroplast RNA polymerase in higher plants. This study identifies a gene for a plastic RNA polymerase sigma factor and indicates that there may be a family of nuclear-encoded sigma factors that recognize promoters in subsets of plastic genes and regulate differential gene expression at the transcriptional level.