Members of the sigma70 family of sigma factors are components of the RNA polymerase holoenzyme that direct bacterial or plastid core RNA polymerase to specific promoter elements that are situated 10 and 35 base-pairs upstream of transcription-initiation points (Paget et al., 2003).  Typical SIG70 type factors are composed of four conserved regions which together enable the basic sigma functions required for promoter and core binding

Chlamydomonas reinhardtii encodes a single sigma70-like factor which functions in chloroplast transcription (Bohne et al., 2006). Plant plastidal sigma factors are encoded in the nuclear genomes of higher plants, and have an N-terminal transit peptide of 30-60 amino acids for transit through the cytoplasm to the that is cleaved off after delivery into the chloroplast.

Plant sigma factors contain a region, essential for transcription, that putatively binds DNA non-template strand downstream -10 box in a sequence specific manner, another DNA binding region that participates in promoter melting without a template, and a region that binds to template strand near the -10 element of the promoter. Plant sigma factors are well described in the literature, and there is evidence that they are all expressed and not pseudogenes (Lysenko 2007). Using two Arabidopsis T-DNA insertion mutants, it was shown that sigma factor 1 (AtSIG1) initiates transcription of a specific subset of chloroplast genes. Also, the photosynthetic control of the PSI reaction center gene transcription requires complementary regulation of the nuclear AtSIG1 gene at the transcriptional level. This AtSIG1 gene regulation is dependent on both a plastid redox signal and a light signal transduced by the phytochrome photoreceptor (Macadlo et al., 2020). The light-dependent inhibition of PHYTOCHROME-INTERACTING transcription FACTORs (PIFs) activates plastid photosynthesis genes via sigma factors as anterograde signals in parallel with the induction of nuclear photosynthesis genes (Hwang et al., 2022).

There are at least 7 sigma70 family members in the maize genome. The first two of these that were studied were designated sig1 and sig2. Transcription of sig1 and sig2 is light inducible and tissue specific. Transcripts of sig1 and sig2 were abundant in greening leaf tissues; sig2 (but not sig1) was barely detectable in etiolated leaves and neither was detectable in roots (Tan et al., 1999).

Last updated June 2023 by John Gray

References:

Paget MS, Helmann JD. The sigma70 family of sigma factors. Genome Biol. 2003;4(1):203. doi: 10.1186/gb-2003-4-1-203. Epub 2003 Jan 3. PMID: 12540296; PMCID: PMC151288.

Bohne AV, Irihimovitch V, Weihe A, Stern DB. Chlamydomonas reinhardtii encodes a single sigma70-like factor which likely functions in chloroplast transcription. Curr Genet. 2006 May;49(5):333-40. doi: 10.1007/s00294-006-0060-7. Epub 2006 Feb 2. PMID: 16453112.

Lysenko EA. Plant sigma factors and their role in plastid transcription. Plant Cell Rep. 2007 Jul;26(7):845-59. doi: 10.1007/s00299-007-0318-7. Epub 2007 Mar 14. PMID: 17356883.

Macadlo LA, Ibrahim IM, Puthiyaveetil S. Sigma factor 1 in chloroplast gene transcription and photosynthetic light acclimation. J Exp Bot. 2020 Jan 23;71(3):1029-1038. doi: 10.1093/jxb/erz464. PMID: 31639823; PMCID: PMC6977190.

Tan S, Troxler RF. Characterization of two chloroplast RNA polymerase sigma factors from Zea mays: photoregulation and differential expression. Proc Natl Acad Sci U S A. 1999 Apr 27;96(9):5316-21. doi: 10.1073/pnas.96.9.5316. PMID: 10220463; PMCID: PMC21861.

Hwang Y, Han S, Yoo CY, Hong L, You C, Le BH, Shi H, Zhong S, Hoecker U, Chen X, Chen M. Anterograde signaling controls plastid transcription via sigma factors separately from nuclear photosynthesis genes. Nat Commun. 2022 Dec 2;13(1):7440. doi: 10.1038/s41467-022-35080-0. PMID: 36460634; PMCID: PMC9718756.