ABSTRACT: Background We previously described the first respiratory Saccharomyces cerevisiae strain, KOY.TM6*P, by integrating the gene encoding a chimeric hexose transporter, Tm6*, into the genome of an hxt null yeast. Subsequently we demonstrated the transferability of this respiratory phenotype in the presence of up to 100 g/L glucose to a yeast strain in which only HXT1-7 had been deleted. In this study, we wanted to examine the basis of the respiratory phenotype of the resultant strain, V5.TM6*P, by comparing its transcriptome with that of its parent, V5, at different glucose concentrations. Results cDNA array analyses revealed that alterations in gene expression that occur when transitioning from a respiro-fermentative (V5) to a respiratory (V5.TM6*P) strain, are very similar to those in cells undergoing a diauxic shift. Highly complete collections of known genes of the TCA cycle, glyoxylate cycle and respiratory chain were identified, consistent with a respiratory metabolism. We also undertook an analysis of transcription factor binding sites in our dataset by examining previously-published biological data for Hap4, Cat8 and Mig1, and using this in combination with verified binding consensus sequences, to identify genes likely to be regulated by one or more of these transcription factors. Of the induced genes of our dataset, 77 % had binding sites for Hap2/3/5 (Hap4 is an activator of this complex), with 72 % having at least two (the latter set being more induced than the former). This is relevant since Hap4 is known to be involved in the transcriptional activation of respiratory genes and other mitochondrial functions. In addition, 13 % of genes were found to have a binding site for Cat8, which together with its complexes with Sip4 have previously been identified as mediating de-repression of a number of genes during the diauxic shift. Finally, 21 % of genes had a binding site for Mig1 which is a transcriptional repressor involved in glucose repression. Unexpectedly, both the up- and down-regulation of many of the genes in our dataset had a clear glucose dependence in the parent V5 strain that was not present in V5.TM6*P. This important result indicates that the relief of glucose repression is already operable at much higher glucose concentrations than is widely accepted and suggests that glucose sensing might occur inside the cell. Conclusions Our dataset gives a remarkably complete view of the involvement of genes in the TCA cycle, glyoxylate cycle and respiratory chain in the expression of the phenotype of V5.TM6*P. Furthermore, 88 % of the transcriptional response of the induced genes in our dataset can be related to the potential activities of just three transcription factors; Hap2/3/5, Cat8 and Mig1. Overall, our data support genetic remodelling in V5.TM6*P consistent with a respiratory metabolism which is insensitive to external glucose concentrations.
