Characterization of Arabidopsis Plastid Sigma-like Transcription Factors SIG1, SIG2 and SIG3

The plastid genome is transcribed by nucleus-encoded (NEP) and plastid-encoded (PEP) RNA polymerases. PEP is a prokaryotic-type enzyme whose activity is regulated by sigma-like transcription initiation factors that are nucleus-encoded. cDNAs coding for six different potential a-like factors have been cloned and sequenced recently. However, functional analyses of these factors are still limited. We have used an anti-sense approach in order to study the function of SIG1, SIG2 and SIG3. Only SIG2 anti-sense plants show a visible phenotype characterized by chlorophyll deficiency. Surprisingly, this phenotype is different from the phenotype of SIG2 knockout plants in that the chlorophyll deficiency is limited to cotyledons. In later developmental stages, the SIG2 anti-sense plants can overcome SIG2 mRNA under-expression by adjusting SIG2 protein levels to that of wild-type plants, suggesting that SIG2 expression is also regulated at the post-transcriptional level. The efficient recovery of the wild-type phenotype could also be supported by partial take-over of SIG2 function by one of the six other sigma factors. A good candidate for such substitution of SIG2 function represents SIG3. SIG3 is constitutively expressed during plant development and its specificity in promoter discrimination is less pronounced than that of SIG1 and SIG2. Finally, SIG3 protein is enhanced in SIG2 anti-sense plants when compared to wild-type plants. SIG2 is present as a soluble factor while SIG3 is partly attached to the plastid membranes. We suggest that membrane localization is necessary for efficient SIG3 function. Therefore, SIG3 cannot substitute for SIG2 function in early chloroplast biogenesis, when plastid membranes are not yet made up.