SEQUENCE AND EXPRESSION CHARACTERISTICS OF A NUCLEAR-ENCODED CHLOROPLAST SIGMA-FACTOR FROM MUSTARD (SINAPIS-ALBA)

Plant chloroplasts contain transcription factors that functionally res emble bacterial sigma factors. We have cloned the full-length cDNA fro m mustard (Sinapis abba) for a 53 kDa derived polypeptide that contain s similarity to regions 1.2-4.2 of sigma(70)-type factors. The amino a cid sequence at the N-terminus has characteristics of a chloroplast tr ansit peptide. An in vitro synthesized polypeptide containing this reg ion was shown to be imported into the chloroplast and processed. The r ecombinant factor lacking the N-terminal extension was expressed in Es cherichia coil and purified. It confers the ability on E.coli core RNA polymerase to bind specifically to a DNA fragment that contains the c hloroplast psbA promoter. Transcription of the psbA template by E.coli core enzyme in the presence of recombinant SIG1 results in enhanced f ormation 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 mustar d. RNA gel blot hybridization reveals a transcript at similar to 1.8 k b, which is more abundant in light-grown than in dark-grown mustard se edlings.