Evolutionary conservation of C-terminal domains of primary Sigma(70)-type transcription factors between plants and bacteria

Three different cDNAs coding for putative plant plastid sigma(70)-type tran scription initiation factors have recently been cloned and sequenced from A rabidopsis thaliana, We have analyzed the evolutionary conservation of func tion(s) of the N-terminal and C-terminal halves of these three sigma factor s by in in vitro transcription studies using heterologous transcription sys tems and by complementation assays using Escherichia coil thermosensitive r poD mutants. Our results indicate differences and similarities of the three plant factors and their prokaryotic ancestors. The functions of the N-term inal parts of the plant sigma factors are considerably different from the f unction of the N-terminal part of the principal sigma(70) factor off. coil, On the other hand, the C-terminal parts have kept at least two characteris tics when compared with their prokaryotic ancestors: 1) they can distinguis h between different promoter structures, and 2) one of them is capable of f ully complementing E. coli rpoD mutants, i,e. recognizing all essential E, coil promoters that are used by the E. coli principal sigma(70) factor. Thi s shows for the first time in vivo a strong evolutionary conservation of ci s- and trans-acting elements between the prokaryotic and the plant plastid transcriptional machinery.