A new protein kinase, SSP31, modulating the SMP3 gene-product involved in plasmid maintenance in Saccharomyces cerevisiae.

The pleiotropic smp3 mutation on the Saccharomyces cerevisiae chromosome confers three phenotypes, (i) stable maintenance of a heterologous plasmid pSR1 isolated from Zygosaccharomyces rouxii, (ii) slow cell growth, and (iii) temperature-sensitive growth. The SSP31 gene of S. cerevisiae was isolated as a chromosomal DNA fragment suppressing the thermosensitive cell growth of the smp3 mutant when it was ligated into the low-copy vector, YCp50. However, it did not suppress the phenotypes of stable maintenance of pSR1 and slow cell growth of the smp3 mutant, even when ligated into the multicopy vector, YEp24. The nucleotide sequence of SSP31 suggested that it encodes a protein of 148 kDa containing all the conserved domains of protein kinases. The putative SSP31 protein also has a hybrid structure of serine/threonine and tyrosine kinases in the diagnostic subdomains for target specificity of protein kinases. Disruption of SSP31 was not lethal, but resulted in slow cell growth. The SSP31 was mapped between the ilv3 and ura2 loci on chromosome X.