Chlamydomonas
635
the mutant nuclear background has been
confrmed. Recently, the genes that are
defcient in these two mutants have been
cloned and characterized. In both cases,
they encode proteins consisting mainly oF
9–10 TPR (tetratricopeptide) repeats that
are present in a wide range oF proteins
with distinct biological Function and that
appear to play an important role in pro-
tein–protein interactions.
A molecular genetic analysis is also Feasi-
ble at the level oF RNA processing or splic-
ing. The
psaA
gene, encoding one oF the
reaction center subunits oF photosystem
I, consists oF three exons that are widely
scattered on the chloroplast genome oF
C. reinhardtii
. Each exon is flanked by
sequences characteristic oF group II in-
trons, and maturation oF the
psaA
message
depends on two trans-splicing reactions.
Surprisingly, intron 1 itselF has a tripartite
structure: the 5
0
part is Fused to exon 1;
the middle part is encoded by a locus,
tscA
,
tha
tisd
is
tan
tF
rombo
thexons1and2
;
and the 3
0
part is Fused to exon 2 (±ig. 7).
The second
psaA
intron has a bipartite
3
i2
psaA-3
5
i1
psaA-1
5
i2
psaA-2
3
i1
tscA
3
5
5
3
5
5
i1
5
i2
3
i1
3
i2
I
II
III
VI
IV
V
Exon 1
Exon 2
Exon 2
Exon 3
I
VI
V
IV
III
II
tscA
Fig. 7
Maturation of the chloroplast
psaA
mRNA of
C. reinhardtii
. Upper part: the three
exons
psaA-1
,-
2
,-
3
and their flanking group II
intron sequences (5
0
i1, 3
0
i1, 5
0
i2, 3
0
i2) and
tscA
(not drawn to scale) are shown on the circular
chloroplast genome. Arrows indicate the
direction of transcription, and the two open
boxes represent the two segments of the
chloroplast inverted repeat. Lower part:
secondary structure of the split-group II intron of
psaA
showing the six characteristic domains.
Intron 1 (left) has a tripartite structure: the
tscA
RNA (heavy line) base-pairs with the 5
0
region of
intron 1 within domain I, completes the catalytic
core by forming domains II and III, and pairs
with the 3
0
part of intron 1 within domain IV.
Intron 2 (right) has a bipartite structure; pairing
occurs within domain IV.
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