Arias et al. BMC Genomics 2011, 12:390 http://www.biomedcentral.com/1471-2164/12/390 Page 11 of 17 Figure 6 Examination of transcriptional activation and Slt2 phosphorylation in mutants selected from the screening. Bars correspond to levels of MLP1-lacZ (a) and CWP1-lacZ (b) expression observed in the indicated deletion mutants and the wild-type strain. Three independent experiments were carried out to calculate the means of b-galactosidase activity and standard deviations. The line graph represents the fold- change in Slt2 phosphorylation for each mutant relative to the values observed in the wild-type strain (1x) as described in Table 1. 2D scatter plots of fold-change in phospho-Slt2 vs transcriptional activation are shown on the left side, including the Pearson’s correlation coefficient (r). response observed in the pmt2Δ and ssd1Δ strains, we took advantage of two mutant forms of MAPK Slt2. The first was a variant K54R, consisting of a mutation within the ATP-binding site, which blocks the catalytic activity of the protein. The second one (TA/YF) eliminates the phosphorylation of Slt2 by upstream MAPKKs Mkk1/ Mkk2. Both alleles, borne on centromeric plasmids, were unable to restore MLP1P-lacZ expression in pmt2Δ slt2Δ and ssd1Δ slt2Δ strains (Figure 8b), indicating that sig- naling through active Slt2 was imperative. These results sustain a mechanism of MAPK signaling in which high levels of transcriptional induction through Rlm1 are not necessarily associated with MAPK phosphorylation levels. This is relevant for CWI pathway-related studies since the phosphorylation status of Slt2 might not always reflect the real pathway outcomes. Further studies will be necessary to characterize the mechanism involved. Conclusions The fine and specific tuning of transduction pathways to ensure yeast cell survival under adverse environmental conditions is essential. Our study contributes significantly to a better understanding of how yeast cell responses to those conditions that jeopardize cell wall integrity or alter its regulation through the CWI pathway. This work has allowed us to identify, at genomic scale, a cluster of genes whose absence induces the transcriptional activation asso- ciated with the cell wall integrity compensatory mechan- ism. Increased levels of phosphorylated MAPK Slt2 were found in a large group of these mutants, in agreement with their direct or indirect association with cell integrity. In fact, the main cluster of genes within this group is related to cell wall biogenesis, morphogenesis and signal transduction. Of special interest are those genes detected by the screening that have not previously been involved in