Disruption of plastid-encoded RNA polymerase genes in tobacco: expression of only a distinct set of genes is not based on selective transcription of the plastid chromosome
K. Krause et al., Disruption of plastid-encoded RNA polymerase genes in tobacco: expression of only a distinct set of genes is not based on selective transcription of the plastid chromosome, MOL G GENET, 263(6), 2000, pp. 1022-1030
Plastids of higher plants operate with at least two distinct DNA-dependent
RNA polymerases, which are encoded in the organelle (PEP) and in the nucleu
s (NEP), respectively. Plastid run-on assays and Northern analyses were emp
loyed to analyse gene expression in tobacco mutant plastids lacking the PEP
genes rpoA, rpoB or rpoC1. Hybridisation of run-on transcripts to restrict
ion fragments representing the entire tobacco plastid chromosome, as well a
s to selected plastid gene-specific probes, shows that all parts of the pla
stid DNA are transcribed in rpo-deficient plastids. In comparison to wild-t
ype chloroplasts, which are characterized by preferential transcription of
photosynthesis-related genes in the light, mutant plastids exhibit a differ
ent transcription pattern with less pronounced differences in the hybridisa
tion intensities between the individual genes. The analysis of steady-state
transcript patterns and transcription rates of selected genes in both type
s of plastids demonstrates that differences in transcription rates are not
necessarily paralleled by corresponding changes in transcript levels. The a
ccumulation of large transcripts in the mutant plastids indicates that proc
essing of primary transcripts may be impaired in the absence of PEP. These
data suggest that, contrary to the prevailing view, much of the regulation
of NEP-driven plastid gene expression in the rpo-deficient mutants is not b
ased on differential promoter usage but is exerted at post-transcriptional
levels.