A Plastid Sigma Factor Sequence from the C₄ Monocot Sorghum bicolor

Abstract

All plastomes of land plants and alga analyzed to date encode homologues of the core subunits of an eubacteria-like RNA polymerase. However, the plastomes lack genes for sigma factors which are imperative for proper promoter recognition by this type of RNA polymerase and hence must be encoded by nuclear genes. By searching plant “expressed sequence tags” (ESTs) for similarities to eubacterial sigma factors an Arabidopsis EST (155H23T7) was identified which was used for the isolation of a homologous full-length cDNA from the C₄ monocot, Sorghum bicolor. The cDNA sequence encodes a protein exhibiting the typical sequence motives of σ⁷⁰-type sigma factors and the corresponding gene was therefore designated Sig1. The SIG1 protein shows the greatest similarity to recently identified plastidial sigma factors of A. thaliana, O. sativa and S. alba. Like these proteins the Sorghum SIG1 protein 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 and accumulate in a light-dependent manner. Sig1 transcript levels are similar in the mesophyll and bundlesheath cells suggesting that at least this sigma 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.

Key words

Sigma factors - C₄ photosynthesis - Sorghum bicolor - plastid transcription