Abstract
Nuclear-encoded sigma factors confer promoter specificity upon the bacterial-type plastid RNA polymerase (PEP). We recently demonstrated that AtSIG5 is a unique sigma factor responsible for the plastid-encoded psbD LRP transcription. Here we report the effect of overexpression of AtSIG5 on plastid transcription using nuclear and chloroplast transformants. Overexpression of AtSIG5 exclusively increased the accumulation of psbD LRP transcripts in the nuclear transformants. To obtain higher levels of AtSIG5 expression, we introduced the truncated AtSIG5 cDNA lacking the putative transit peptide in plastid genome of tobacco. As expected, hyperexpression of AtSIG5 in chloroplasts led much higher level of the psbD LRP transcripts. Interestingly, transplastomic plants exhibited a bleached phenotype, whereas the nuclear transformants were normal. In the chloroplat transformants, psbD, psaA and ndhD transcripts increased, whereas the expression of some PEP-dependent genes, including psbB and rrn23 decreased drastically. Chloroplast transformation would provide a powerful tool to characterize nuclear-encoded chloroplast proteins.
