Dosage suppressors of the dominant G1 cyclin mutant CLN3-2: identification of a yeast gene encoding a putative RNA/ssDNA binding protein.

Three G1 cyclins, CLN1, CLN2, and CLN3, have been identified in the budding yeast Saccharomyces cerevisiae. G1 cyclins are essential, albeit functionally redundant, rate-limiting activators of cell cycle initiation. We have isolated dosage-dependent suppressor genes (designated HMD genes) of the mating defect caused by CLN3-2, a dominant mutation in CLN3, HMD2 and HMD3 are identical to STE4 and STE5, respectively, HMD1 is an essential gene that encodes a protein containing a putative RNA binding domain. Overproduction of HMD1 results in a relatively specific reduction in the level of the CLN3 or CLN3-2 transcript. This reduction occurs subsequent to transcription initiation of CLN3 since overexpression of HMD1 did not affect expression of a heterologous transcript from the CLN3 promoter but did result in a reduction of CLN3 transcript expressed from a heterologous promoter. HMD1 has at least one essential role independent of its effect on CLN3 since HMD1 remains essential for viability in the absence of a functional CLN3 gene.