A plastid sigma factor sequence from the C-4 monocot Sorghum bicolor

All plastomes of land plants and alga analyzed to date encode homologues of the core subunits of an eubacteria-like RNA polymerase. However, the plast omes lack genes for sigma factors which are imperative for proper promoter recognition by this type of RNA polymerase and hence must be encoded by nuc lear genes. By searching plant "expressed sequence tags" (ESTs) for similar ities to eubacterial sigma factors an Arabidopsis EST (155H23T7) was identi fied which was used for the isolation of a homologous full-length cDNA from the C-4 monocot, Sorghum bicolor. The cDNA sequence encodes a protein exhi biting the typical sequence motives of sigma(70)-type sigma factors and the corresponding gene was therefore designated Sig1. The SIG1 protein shows t he greatest similarity to recently identified plastidial sigma factors of A . thaliana, O. sativa and S. alba. Like these proteins the Sorghum SIG1 pro tein contains an aminoterminal extension reminiscent of plastidial transit sequences indicating that the SIG1 protein functions as a plastidial sigma factor. Accordingly, Sig1 transcripts are found preferentially in leaves an d accumulate in a light-dependent manner. Sig1 transcript levels are simila r in the mesophyll and bundle-sheath cells suggesting that at least this si gma factor is not involved in the differential expression of plastid genes in the two cell types. Southern analyses indicate that the Sig1 sequence is present as single copy gene in the Sorghum genome.