Dbf2, a cell cycle-regulated protein kinase, has been shown recently to be part of the CCR4 transcriptional regulatory complex in the yeast Saccharomyces cerevisiae. We report here that temperature-sensitive (ts) dhf2-2 mutant cells can be rescued by overexpression of CDC14, which encodes a dual-specificity protein phosphatase, when grown on glucose-containing medium, as reported previously, but not on galactose. Screening of two S. cerevisiae cDNA libraries led to the identification of CBT1 as a gene which, if overexpressed simultaneously with CDC14, results in the rescue of the dbJ2-2 mutation at restrictive temperature on galactose-based medium, as well as on media containing non-fermentable carbon sources such as glycerol, lactate and acetate. Cbt1 has been shown previously to be essential for formation of cytochrome b in the mitochondria. On the other hand, the ts defects of ccr4delta and caf1delta mutants on glycerol medium at 37 degrees C (Ccr4 and Caf1/Pop2 are two other members of the CCR4 complex) could not be complemented by simultaneous overexpression of CBT1 and CDC14. CCR4 and CAF1 have been shown to play an essential role in activating the expression of genes for non-fermentative growth. Our results strongly suggest that, within the CCR4 complex, Dbf2 is directly involved in some mitochondrial function that depends on cytochrome b or on one of its main regulators, Cbt1. Therefore, Dbf2 may be required not only during mitosis but also during growth on non-fermentable carbon sources.