Untitled

Abstract

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.

Nucleus-encoded plastid sigma factor SIG3 transcribes specifically the psb N gene in plastids

“…The same holds true for other sigma factor/promoter interactions such as SIG2/ psa J (25), SIG2/ trn E-V-M and Q (24), SIG4/ ndh F (31) and SIG5/ psb D BLRD (27). At a first glance, the specificity of SIG3 reported here for the psb N gene promoter seems to be in contradiction with previous publications showing recognition of the psb A and rbc L promoters by SIG3 (15,23). However, these experiments have been made using in vitro transcription conditions without sigma competition.…”

“…However, these experiments have been made using in vitro transcription conditions without sigma competition. Similar results showing sigma specificity in vivo under competitive conditions and apparently less specificity under non-competitive conditions are also reported for SIG2 with respect to the psb A promoter (15,23,25,27). In addition to redundant action of sigma factors under non-competitive conditions we can also not exclude that additional SIG3-specific promoters exist in the multiple promoter regions that precede many of the plastid transcription units.…”

“…In general, it seems that SLFs have overlapping as well as specific functions (15,22,23). Although overlapping functions have been demonstrated by in vitro transcription assays that are performed without competition by other sigma factors, the specific functions are more easily detected by analyses that reflect in vivo competition conditions, i.e.…”

Blue light-induced transcription of plastid-encoded psbD gene is mediated by a nuclear-encoded transcription initiation factor, AtSig5

“…On the other hand, the transient expression of AtSig2 specifically activated transcription of trnEYD operon and psbA in the dark-adapted protoplasts. These results concur with previous findings that AtSIG2 deficiency reduced the accumulation of some plastid-encoded tRNAs (29) and psbA transcripts (30). Similarly, the accumulation of psbD BLRP transcripts also depended on seedling growth.…”

“…The most characterized plastid factor to date is AtSig2. Deficiency of AtSIG2 resulted in reduced accumulation of several tRNAs including trnE in Arabidopsis (29,30 On the other hand, this work demonstrated that AtSig5 is specifically required for psbD BLRP activity. AtSig5 is also involved in the transcription of psbA, and possibly psaA and psbB genes.…”