Deletion of SFI1, a novel suppressor of partial Ras-cAMP pathway deficiency in the yeast Saccharomyces cerevisiae, causes G(2) arrest

When glucose is added to Saccharomyces cerevisiae cells grown into stationa ry phase or on non-fermentable carbon sources a rapid loss of heat stress r esistance occurs. Mutants that retain high stress resistance after addition of glucose are called 'fil', for deficient in fermentation Induced loss of stress resistance. Transformation of the fill mutant, which harbours a poi nt mutation in adenylate cyclase, with a yeast gene library on a single cop y plasmid resulted in transformants that were again stress-sensitive. One o f the genes isolated in this way was a gene of previously unknown function. We have called it SFI1, for suppressor of fil1. SFI1 is an essential gene. Combination of Sfi1 and cAMP pathway mutations indicates that Sfi1 itself is not involved in the cAMP pathway. Conditional sfi1 mutants did not show enhanced heat resistance under the restrictive condition, whereas overexpre ssion of SFI1 rendered cells heat-sensitive. Sfi1 may be a downstream targe t of the protein kinase A pathway, but its precise relationship with heat r esistance remains unclear. Further analysis showed that Sfi1 is required fo r cell cycle progression, more specifically for progression through G(2)-M transition. Cells expressing SFI1 under the control of a galactose-inducibl e promoter arrest after addition of glucose as doublets of undivided mother and daughter cells. These doublets contain a single nucleus and lack mitot ic spindles. Sfi1 shares homology with Xenopus laevis XCAP-C, a protein req uired for chromosome assembly. The conserved residues between these two pro teins show a strong bias for charged amino acids. Hence, Sfi1 might be requ ired for correct mitotic spindle assembly and its precise role might be in chromosome condensation. In conclusion, we have identified an essential fun ction in the G(2)-M transition of the cell cycle for a yeast gene of previo usly unknown function. The EMBL Accession No. of the SFI1 nucleotide sequen ce is X95569. Copyright (C) 1999 John Wiley & Sons, Ltd.