A gene with an open reading frame encoding a protein of 417 amino acid residues with a Gly-Thr repeat was isolated from the yeast Saccharomyces cerevisiae by using synthetic oligonucleotides encoding three Gly-Thr dimers as probes. The deduced amino acid sequence showed partial homology to the clock-affecting gene, per, of Drosophila melanogaster in the regions including the GT repeat. The function of the gene, named GTS1, was examined by characterizing the phenotypes of transformants with different copy numbers of the GTS1 gene produced either by inactivating the GTS1 gene by gene disruption (TMAgtsl) or by transformation with multicopy plasmid pPER119 (TMpGTS1). They grew at similar rates during the exponential growth phase, but the lag phases were shorter for TMAgtsl and longer for TMpGTS1 cells than that for the wild type. Analyses of their cell cycle parameters using synchronized cells revealed that the unbudding period changed as a function of gene dosage; that is, the periods of TMAgtsl and TMpGTS1 were about 20%o shorter and longer, respectively, than that of the wild-type. Another significant change in the transformants was detected in the distribution of the cell size. The mean cell volume of the TMAgtsl cells in the unbudded period (single cells) was 27% smaller than that of single wild-type cells, whereas that of single TMpGTS1 cells was 48% larger. Furthermore, in the temperature-sensitive cdc4 mutant, the GTS1 gene affected the timing of budding at the restrictive temperature. Thus, the GTS1 gene product appears to modulate the timing of budding to obtain an appropriate cell size independent of the DNA replication cycle.Among the genes known to affect biological clocks, the period gene (per) of Drosophila melanogaster (24, 37) and the frequency gene (frq) of Neurospora crassa (31) were the first to be isolated and sequenced. Characteristically, the amino acid sequences deduced from the nucleotide sequences of both genes each contained a Gly-Thr/Ser repeat which is shared by proteoglycans comprising a polysaccharide attachment site (31,37,43). Although the GT repeats are polymorphic in length and in sequence among the species of Drosophila (10,11,16,34,35,48,51), the presence of the GT repeat in the clockaffecting genes from evolutionarily remote species, Drosophila and Neurospora species, implies that this sequence plays an important role in most, if not all, clock-affecting proteins.The nature of per mutant phenotypes suggested that the gene product is an integral component of the circadian clock: mutations either shorten (pers), lengthen (per'), or abolish (per0) not only the period of circadian rhythms (19) but also the rhythm with a much shorter period during the male courtship song (26,52). Furthermore, the period of the circadian rhythm of D. melanogaster changes depending upon the gene dose (4,12,45