Efficient splicing in vivo of most self-splicing group I
introns is believed to require proteins, raising the possibility
that splicing could be regulated; however, examples of
such regulation have been lacking. The Chlamydomonas
reinhardtii chloroplast psbA gene contains
four large group I introns that self-splice efficiently
in vitro, but only under nonphysiological conditions. The
psbA gene encodes the D1 protein of photosystem
II, which is synthesized at very high rates in the light
in order to replace photodamaged protein. We show that
psbA pre-mRNAs, containing one or more introns,
accumulate in wild-type cells in the dark, apparently due
to rate-limited splicing. Analysis of the pre-RNAs indicates
that splicing of the four introns does not follow a strict
order. Exposure of cells to light induced rapid (15–20
min) decreases in precursor levels of ∼3–5-fold
(depending on the intron), which were accompanied by transient
increases in free intron levels. Because light also stimulated
psbA transcription ∼2-fold over the same period,
the data suggests that light increases the splicing efficiency
of psbA introns ∼6–10-fold. Similar
estimates of the extent of light stimulation were obtained
by analyzing precursor decay rates in the presence of actinomycin
D. The effect of light is specific for psbA introns,
because levels of unspliced 23S pre-RNA did not decrease.
The light-induced increase in psbA pre-RNA processing
was abolished by inhibitors of photosynthetic electron
transport, but not by the ATP synthesis inhibitor, carbonylcyanide
m-chlorophenylhydrazone, which actually promoted
pre-RNA processing in the dark. Finally, nonphotosynthetic
mutants, including the tscA-lacking photosystem
I mutant, H13, did not show evidence of light-stimulated
RNA processing. However, the light response was restored
in photosynthetic transformants of H13 that had been complemented
with the tscA gene. These data suggest strongly
that light coordinately stimulates splicing of all four
psbA introns. Moreover, they demonstrate that
this response to light is mediated by photosynthetic electron
transport. The implications of these results for the regulation
of psbA gene expression are discussed.