Chlamydomonas
627
The
tuf
A gene encoding EF-Tu is lo-
cated in the chloroplast genome in
C.
reinhardtii
and in the nuclear genome in all
higher and lower plants examined. Three
chloroplast genes encoding subunits of the
enzyme involved in light-independent pro-
tochlorophyllide reduction are present in
C. reinhardtii
but not in higher plants. In-
terestingly, these genes are also present
in the chloroplasts of lower plants and
pines that, together with
C. reinhardtii
,
are capable of synthesizing chlorophyll in
theabsenceo
fl
igh
t
.F
ina
l
ly
,afewopen
reading frames (ORFs) of unknown func-
tion and unique to
C. reinhardtii
have been
identi±ed. Chloroplast genes that are ab-
sent from
C. reinhardtii
, but present in land
plants include those encoding several ribo-
somal proteins and subunits of the NADH
dehydrogenase. This enzyme is thought
to mediate electron transfer from reduced
stromal components to the plastoquinone
pool of the photosynthetic electron transfer
chain.
The chloroplast DNA is associated with
small basic proteins and appears to be
under torsional stress. Both relaxing and
supercoiling topoisomerases have been
detected in the chloroplast, and inhibi-
tion of the latter with novobiocin re-
duces torsional stress and alters chloro-
plast transcription
in vivo
. As in several
prokaryotic organisms, the chloroplast
DNA–dependent RNA polymerase of
C.
reinhardtii
is sensitive to rifampicin. Some
chloroplast promoters resemble typical
prokaryotic promoters with characteristic
10 and
35 boxes. However, several
chloroplast promoters lack the
35 box
and can be as short as 22 bp. Transcrip-
tion rates of chloroplast genes vary almost
100-fold. RNA stability can also vary con-
siderably from one transcript to the other
and, for all transcripts tested, the stabil-
ity is signi±cantly higher in dark- than
in light-grown cells. Hence, changes in
the abundance of chloroplast transcripts
are regulated in a gene-speci±c manner
and depend on both environmental and
endogenous factors.
Splicing is an important posttranscrip-
tional step in chloroplast gene expression.
Most chloroplast introns fall into two ma-
jor classes, group I and group II, ±rst
identi±ed in yeast mitochondrial genes.
Members of each class possess a char-
acteristic secondary structure. Whereas
introns are present in many chloroplast
genes from plants, only three chloroplast
genes from
C. reinhardtii
contain introns.
Group I introns occur in the 23S riboso-
mal RNA and
psb
A genes of
C. reinhardtii
.
Most of these highly structured introns
areab
letose
lf
-sp
lice
in vitro
.Anotherun
-
usual property of some of these group
I introns is their ability to move at the
DNA level, provided a suitable integration
site is available. This process, called
in-
tron homing
, was ±rst discovered in yeast
mitochondria. In crosses between intron-
containing and intronless parents, all the
progeny inherit the intron. Intron homing
hasa
lsobeenfoundtooccurinbac
ter
io
-
phage T4, in the slime mold
Physarum
,
and in the chloroplast and mitochon-
dria of
Chlamydomonas
.A
l
lth
em
o
b
i
l
e
group I introns contain an ORF encod-
ing a protein with double-stranded DNA
endonuclease activity that cleaves the in-
tronless allele at the site at which the
intron is inserted. Cleavage triggers trans-
position of an intron copy to this target
site and leads to ef±cient spreading of
the intron to all target sites available. Be-
sides cis-splicing there is trans-splicing in
the chloroplast of
C. reinhardtii
during
the maturation of the
psa
A message (cf.
Sect. 4.1.1).
The chloroplast translational appara-
tuses of
Chlamydomonas
and plants share
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