Abstract
Protein farnesyltransferase (FTase, E. C. 2.5.1.21)
post-translationally modifies regulatory proteins
involved in controlling cell growth, division, and
differentiation. Recently, a cDNA clone (PsFTb)
encoding a pea (Pisum sativum) FTase b subunit was
isolated. Initial studies led to the hypothesis that FTase
plays a role in the regulation of plant cell division. To
gain insight into FTase function in plants, a detailed
study of the expression pattern of FTase genes was
carried out. A cDNA (NgFTb) encoding the b subunit
of tobacco FTase was cloned from a Nicotiana
glutinosa cDNA library to initiate studies in tobacco.
In tobacco BY-2 suspension culture, levels of NgFTb
mRNA and FTase activity transiently increased at the
early log phase of cell growth and rapidly declined
before cells entered stationary phase. These data,
along with inhibitor studies in the BY-2 system,
support our hypothesis. To understand the expression
and regulation of pea FTase subunit genes,
5'-upstream sequences of both pea FTase subunit
genes (PsFTb and PsFTa) were cloned from a pea
genomic library. The 5'-upstream sequence (~2 kb)
of PsFTa was fused to GUS (b-glucuronidase) and
GFP (green fluorescent protein) reporter genes and
introduced into tobacco plants. This 2 kb upstream
region appears insufficient to provide PsFTa promoter
function. On the other hand, 3.2 kb of PsFTb
5'-upstream sequence expressed as a PsFTb:GUS
construct is fully functional in transgenic tobacco
plants. GUS expression was most prominent in
actively growing cells supporting FTase involvement in
plant cell cycle control. GUS activity was also found
in mature and imbibed embryos but not premature
embryos, consistent with the role of FTase in abscisic
acid (ABA) signaling. An unexpected pattern of GUS
activity, not correlated with dividing cells or ABA
signaling, was also observed in the transgenic plants.
GUS activity was detected in vascular bundles
adjacent to actively-growing tissues and in regions that
connect two organs, e.g., junctions between stems
and leaf petioles, cotyledons and hypocotyls, roots
and hypocotyls. Auxin promotes PsFTb expression
while light and sucrose inhibit expression. These
spatial and temporal expression patterns strongly
suggest that FTase has a broader role associated with
regulation of nutrient transportation or allocation in
plants.
|
