Plant Physiol, August 2000, Vol. 123, pp. 1517-1524
Fluoroorotic Acid-Selected Nicotiana plumbaginifolia
Cell Lines with a Stable Thymine Starvation Phenotype Have Lost the
Thymine-Regulated Transcriptional Program1
Djoko
Santoso2 and
Robert
Thornburg*
Department of Biochemistry, Biophysics, and Molecular Biology, Iowa
State University, Ames, Iowa 50011
We have selected 143 independent Nicotiana
plumbaginifolia cell lines that survive in the presence of
5-fluoroorotic acid. These lines show several diverse phenotypes. The
majority of these cell lines showed reduced levels of UMP synthase.
However, one particular phenotype, which represents 14% of the total
independent lines (20 cell lines), showed an unexpected, high level of
UMP synthase and was therefore analyzed in detail. The selected cell lines showed no differences with wild-type cells with respect to uptake
of orotic acid, affinity of UMP synthase for its substrates, or UMP
synthase gene-copy number. Alternative detoxification mechanisms were
also excluded. The elevated enzyme activity was correlated with
elevated UMP synthase protein levels as well as elevated UMP synthase
mRNA levels. In contrast to wild-type cell lines, the fluoroorotic
acid-selected cell lines did not respond to thymine or to other
biochemicals that affect thymine levels. In addition, there was also a
concomitant up-regulation of aspartate transcarbamoylase, however, dihydroorotase and dihydroorotate dehydrogenase are not up-regulated in these cell lines.
1
This work was sponsored by the U.S. Department
of Agriculture (grant no. 91-37301-6208). This is journal paper no.
J-16512 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa and project no. 3202.
2
Present address: Biotechnology Research Unit for
Estate Crops, Bogor, Indonesia.
*
Corresponding author; e-mail thorn{at}iastate.edu; fax
515-294-0453.
© 2000 American Society of Plant Physiologists
