Plant chloroplasts contain transcription factors that functionally resemble bacterial sigma factors. We have cloned the full-length cDNA from mustard (Sinapis alba) for a 53 kDa derived polypeptide that contains similarity to regions 1.2-4.2 of sigma70-type factors. The amino acid sequence at the N-terminus has characteristics of a chloroplast transit peptide. An in vitro synthesized polypeptide containing this region was shown to be imported into the chloroplast and processed. The recombinant factor lacking the N-terminal extension was expressed in Escherichia coli and purified. It confers the ability on E.coli core RNA polymerase to bind specifically to a DNA fragment that contains the chloroplast psbA promoter. Transcription of the psbA template by E.coli core enzyme in the presence of recombinant SIG1 results in enhanced formation of transcripts of the size expected for correct initiation at the in vivo start site. Together, these data suggest that the mature protein acts as one of the chloroplast transcription factors in mustard. RNA gel blot hybridization reveals a transcript at approximately 1.8 kb, which is more abundant in light-grown than in dark-grown mustard seedlings.
A 1887-nucleotide chloroplast-DNA region from Arabidopsis thaliana was analyzed. It contains the conserved genes psbA for the precursor of the D1 reaction-centre protein of photosystem II, trnH for tRNAHis, and rps19' for the 6.8-kDa protein of the small ribosomal subunit. Northern hybridization and RNase protection experiments suggest co-transcription of a minor RNA fraction over the full lengths of psbA and the preceding trnK-UUU gene, but not including downstream trnH sequences. In front of the mapped 5' end of the major 1.2-kb psbA transcript is a DNA region that shows the typical architecture of a psbA promoter, consisting of the prokaryotic-type '-35' and '-10' elements as well as the eukaryotic-type 'TATA' motif. The common 3' end of psbA transcripts seems to be located immediately after a stem-loop structure downstream from the coding region.
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