| dc.description.abstract | The psbA gene encodes the 01 protein involved in
electron transport of photosynthesis leading to production of
NAO(P)H and ATP. The ~ gene encodes the large subunit of
the ribulose,-1,5-bisphosphate carboxylase (Rubisco), an
enzyme involved in the initial step of C02 fixation. A large
open reading frame (ORF) of about 2131-2280 amino acids has
been located on several plastid genomes. The objectives of
this study were to determine the patterns of the expression
of the ~, ~ genes and to examine whether the ORF 2280
was expressed in the plastid of three Cucurbita pepo L.
varieties, namely, Early Prolific (EP), Fordhook Zucchini and
Bicolor gourds. These objectives were achieved by isolation
of total RNA from the skins of squash fruits and probing with
.gene-specific and ORF 2280-specific probes. Changes in ~
mRNAs were also compared with variations in Rubisco LSU and
Rubisco activity.
In the three varieties of squash the expression of the
rbcL and psbA genes was developmentally regulated. For
example in EP fruits, both transcripts increased steadily
with fruit development up to 21 days postpollination.
Evidence of post-transcriptional regulation of ~ gene was
observed in Zucchini and Bicolor fruits. In Zucchini, the
amounts of ~ mRNAs increased to a maximum at 3 days and
slightly declined at day 14 and 21. However, the Rubisco
activity at 14 and 23 days significantly exceeded activities
in young fruits.
The expression of the rbcL and psbA genes did not
correlate with the plastid type in EP fruits but correlated
with plastid type in Bicolor fruits. In Bicolor fruits, the
levels of both transcripts were slightly greater in the
chromoplast tissue than in the chloroplast tissue. Rubisco
activity was significantly higher in the chromoplasts than in
the chloroplasts of the same fruit.
Differential gene expression was observed in Bicolor and
Zucchini fruits, where the steady-state amounts of ~ mRNAs
exceeded the expression of the ~ mRNAs. For example, the
steady-state amounts of ~ mRNAs exceeded ~ transcripts
in Fordhook Zucchini by 2.5- to 3.9-fold, while in Bicolor
fruits the amounts of psbA mRNAs were greater than rbcL
transcripts by 3.3- to 4.2-fold. Regulation of gene
expression at the tissue specific level was also observed in
Zucchini and Bicolor fruits and leaves. For example, in
Bicolor and Fordhook Zucchini fruits the amounts of ~
mRNAs in the leaf exceeded the levels in the fruits by a
range of 3.3- to 8-fold, and 2.8- to 59-fold, respectively.
ORF 2280 transcripts of 2.6 kb were detected in EP and
Bicolor fruits, while a 1.4 transcript was also detected in
Bicolor fruits. | en |
