Abstract:
Thioredoxin was initially identified by its ability to serve as an
electron donor for ribonucleotide reductase in vitro. Whether it
serves a similar function in vivo is unclear. In Saccharomyces cerevisiae,
it was previously shown that trx1 trx2 mutants lacking the
two genes for cytosolic thioredoxin have a slower growth rate
because of a longer S phase, but the basis for S phase elongation was
not identified. The hypothesis that S phase protraction was due to
inefficient dNTP synthesis was investigated by measuring dNTP
levels in asynchronous and synchronized wild-type and trx1 trx2
yeast. In contrast to wild-type cells, trx1 trx2 cells were unable to
accumulate or maintain high levels of dNTPs when -factor- or
cdc15-arrested cells were allowed to reenter the cell cycle. At 80 min
after release, when the fraction of cells in S phase was maximal, the
dNTP pools in trx1 trx2 cells were 60% that of wild-type cells.
The data suggest that, in the absence of thioredoxin, cells cannot
support the high rate of dNTP synthesis required for efficient DNA
synthesis during S phase. The results constitute in vivo evidence for
thioredoxin being a physiologically relevant electron donor for
ribonucleotide reductase during DNA precursor synthesis.
