SEQUENCE OF THE HAP3 TRANSCRIPTION FACTOR OF KLUYVEROMYCES-LACTIS PREDICTS THE PRESENCE OF A NOVEL 4-CYSTEINE ZINC-FINGER MOTIF

The Kluyveromyces lactis homologue of the Saccharomyces cerevisiae HAP 3 gene was isolated by functional complementation of the respiratory-d eficient phenotype of the S. cerevisiae hap3::HIS4 strain SHY40. The K lHAP3 gene encodes a protein of 205 amino acids, of which the central B-domain of 90 residues is highly homologous to HAP3 counterparts of S . cerevisiae and higher eukaryotes. The protein contains a novel dr-cy steine zinc-finger motif and we propose by analogy that all other homo logous HAP3 proteins contain the same motif, with the position contain ing the third cysteine being occupied by a serine residue. In contrast to the situation in S. cerevisiae, disruption of the KlHAP3 gene in K . lactis does not result in a respiratory-deficient phenotype and the growth of the null strain is indistinguishable from wild type. There i s also no effect on the expression of the carbon source-regulated KlCY C1 gene, suggesting either a different role for the HAP2/3/4 complex, or the existence of a different mechanism of carbon source regulation. Sequence verification of the S. cerevisiae HAP3 locus reveals that, j ust as in K. lactis, a long open reading frame (ORF) is present upstre am of the HAP3 gene. These highly homologous ORFs are predicted to hav e at least eight membrane-spanning fragments, but do not show signific ant homology to any known sequence present in databases. The ScORFX ge ne is transcribed in the opposite direction to ScHAP3, but, in contras t to an earlier report by Hahn et al. (1988), the transcripts of the t wo genes do not overlap. The model proposed by these authors, in which the ScHAP3 gene is regulated by an anti-sense non-coding mRNA, is the refore not correct.