brevicompactum mpaF (type IMPDH-B). There are 30 residues known to be important for catalytic function and these are completely conserved in all IMPDHs identified at present [1]. All of the 30 residues, except for the one corresponding to position 415 (numbering follows MpaFp), were also conserved in IMPDH-B from both P. chrysogenum and P. brevicompactum. The residue at position 415 is part of the active site and was found to be phenylalanine in both IMPDH-B sequences (Figure 4); whereas

this position is featured by tyrosine in all IMPDH-A type proteins. In addition, when comparing IMPDH-A and IMPDH-B sequences, the so-called IMPDH click here “”flap-region”" [1] is variable including a five-residue-long gap in the two IMPDH-Bs (Figure 4). Although these sequence differences may seem significant, they are not obvious candidates for conferring MPA resistance. The substitution

at position 415 is not in close proximity to the MPA binding site and the sequence of the “”flap-region”" is known to be highly variable and has so far not been linked to MPA sensitivity [16]. Furthermore, P. chrysogenum is not a MPA producer and it is therefore not self-evident that the IMPDH-B from this fungus is resistant. Additional IMPDH sequences from MPA producers and non-producers will be see more useful in the search for the functionally critical residues. Moreover, comparative biochemical characterization of IMPDH-A and IMPDH-B, as well as of mutant derivatives, will be necessary to quantify the degree of resistance,

and to pinpoint the residues important for MPA resistance. Such biochemical characterization, together with the measurement of expression levels of IMPDH-A and IMPDH-B in MPA producers, will help in dissecting the relative contribution of each type to MPA self-resistance. Figure 4 Multiple sequence alignment of selected fungal IMPDHs. The region ROS1 including the amino acid residue at position 415 and part of the flap-region (flap-region being spanned by residues 412 – 467) is presented in the figure. The position 415 is tyrosine in all IMPDHs identified prior to this work [1]. Note that the flap region is very variable, with only residue 415Y and key catalytic residues 441R and 442Y completely conserved in all IMPDHs identified prior to this work [1]. Residues conserved among all nine sequences are highlighted in grey. P. brevicompactum IMPDH-B (encoded by mpaF) is used as a reference while referring to position numbers. P, Penicillium; A, Aspergillus. IMPDH-B has possibly emerged through gene duplication IMPDHs are highly conserved enzymes, which points to their important role in fitness. A high level of conservation was also observed for the sequences obtained from the six Penicillium strains investigated in our study.