The chloroplast gene psbS encodes the chlorophyll-a binding protein P5 (CP4
7), one of the core subunits of photosystem II (PSII). The psbB mRNA and th
e downstream psbT and psbH transcripts fail to accumulate in the Chlamydomo
nas reinhardtii nuclear mutant 222E affected in the Mbb1 gene (Monod et al.
, 1992, Mel. Gen. Genet. 231, 449-459). By introducing chimeric genes consi
sting of sequences from psbB and the reporter gene aadA into the chloroplas
t, the target site of Mbb1 was mapped in the psbB 5' untranslated region (U
TR). Primer extension analysis indicates that the psbS RNA exists in a less
abundant long form and a more abundant short form, with 5' ends at positio
ns -147 and -35 relative to the AUG initiation codon, respectively. The lon
ger transcript is present both in the wild type (WT) and 222E mutant, but t
he shorter one accumulates only in the WT. Two putative stem-loop structure
s in the longer 5' UTR can be deleted individually without affecting psbB m
RNA accumulation. Insertion of a poly G cassette in the long leader stabili
zes a chimeric psbB transcript in the 222E mutant, suggesting the involveme
nt of a 5'-3' exonuclease. We also show that psbH and psbT are transcribed
from the upstream psbB gene promoter, and that the psbH mRNA has its own ta
rget sequence for Mbb1 function. We discuss the role of this nucleus-encode
d factor, required for specific chloroplast gene expression, in the assembl
y of the multi-protein PSII complex.