Abstract
Three different cDNAs coding for putative plant plastid sigma(70)-type transcription initiation factors have recently been cloned and sequenced from Arabidopsis thaliana. We have analyzed the evolutionary conservation of function(s) of the N-terminal and C-terminal halves of these three sigma factors by in vitro transcription studies using heterologous transcription systems and by complementation assays using Escherichia coli thermosensitive rpoD mutants. Our results indicate differences and similarities of the three plant factors and their prokaryotic ancestors. The functions of the N-terminal parts of the plant sigma factors are considerably different from the function of the N-terminal part of the principal sigma(70) factor of E. coli. On the other hand, the C-terminal parts have kept at least two characteristics when compared with their prokaryotic ancestors: 1) they can distinguish between different promoter structures, and 2) one of them is capable of fully complementing E. coli rpoD mutants, i.e. recognizing all essential E. coli promoters that are used by the E. coli principal sigma(70) factor. This shows for the first time in vivo a strong evolutionary conservation of cis- and trans-acting elements between the prokaryotic and the plant plastid transcriptional machinery.
Highlights
The higher plant plastid genome is transcribed by two different transcriptional systems
Overproduction of Entire and Truncated Plant Sigma Factors in E. coli—We have cloned and sequenced the cDNAs corresponding to six putative A. thaliana sigma factors as described under “Materials and Methods.”
We found that the N-terminal part was sufficient to provoke growth arrest of E. coli
Summary
The higher plant plastid genome is transcribed by two different transcriptional systems (reviewed in Ref. 1). Genes corresponding to sigma-like transcription initiation factors that are indispensable for the activity of this type of enzyme (reviewed in Ref. 8) are not found on the plastid genomes [7, 1]. We attempted to obtain information about the evolutionary conservation of components of the transcriptional machinery between prokaryotes and higher plant plastids With this aim, we analyzed differences of the three plant sigma factors with respect to promoter recognition by in vitro transcription, and we determined the plant analogue to the primary sigma factor of E. coli by complementation of E. coli rpoD(ts) mutants. We show in vivo complementation of a protein of the E. coli transcriptional machinery by a protein of the plastid transcriptional machinery
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