Phosphorylation and dephosphorylation affect functional characteristics of chloroplast and etioplast transcription systems from mustard (Sinapis alba L.).

Chloroplast and etioplast RNA polymerase preparations each consist of a multi-subunit core and a set of three sigma-like transcription factors, SLF67, SLF52 and SLF29. Despite this structural similarity, the enzymes from either plastid type are functionally distinct, as is reflected by their different promoter usage and the tight core-SLF association in the etioplast but not the chloroplast holoenzyme. We tested whether these differences are related to phosphorylation. Treatment of the chloroplast enzyme with protein kinase converted it to an etioplast-type form and vice versa, treatment of the etioplast enzyme with phosphatase generated chloroplast-type properties. Although both the core enzyme and the SLF polypeptides were phosphorylation targets, only the SLFs seem to confer plastid-type-specific DNA binding characteristics. Methylation interference and DNase I footprint patterns in the psbA promoter region were found to correlate with the phosphorylation state of the chloroplast and etioplast enzymes.