The Saccharomyces cerevisiae SOC8-1 gene and its relationship to a nucleotide kinase

The yeast SOC8-1 gene was originally identified by partial complementation of cdc8 mutant strains. We have carried out Bal31 deletion analysis of the SOC8-1 gene to define the minimal size which is required for the complementation of the cdc8 mutation. When the SOC8-1 gene is cloned in a multicopy plasmid, it enables temperature-resistant growth in the cdc8 mutant strain, while the SOC8-1 gene in a single copy plasmid does not. Thus, its suppression of the cdc8 mutant is dosage dependent. The high copy number vector carrying the SOC8-1 gene can complement five different cdc8 alleles, indicating that the suppression is not allele specific. Since CDC8 encodes thymidylate kinase, cells bearing a high copy number plasmid containing SOC8-1 gene were tested for the ability to phosphorylate several nucleoside monophosphates, including UMP, GMP and dTMP. Significantly increased phosphorylation activity was observed, suggesting that SOC8-1 encodes a nucleotide kinase. Both restriction enzyme analysis of the SOC8-1 gene and partial purification of the overproduced kinase in SOC8-1 overproducing strains suggest that SOC8-1 may be allelic with URA6. Consistent with these results, both SOC8-1 and URA6 are located on chromosome XI. Thus, one possible suppression mechanism is that SOC8-1 may provide a trans-acting dTMP kinase activity, bypassing the cdc8 gene defect.