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 (PMID:12540296). 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 ?10 element of promoter. Plant sigma factors are well described in the literature, and there is evidence that they are all expressed and not pseudogenes (PMID:17356883).