Biomedical Engineering Reference
In-Depth Information
coordination for optimal cephalosporin formation. The fi rst one, encoded by
the genes located in the “early” cluster, results in the formation of PenN, a
penicillin active against several Gram-positive bacteria. The second (“late”)
cluster includes only two genes encoding enzymes that convert PenN to
cephalosporin C. The second of those enzymes, the DAC-acetyltransferase
(encoded by the
cefG
), is a typical eucaryotic acetyltransferase that is not
present in cephamycin-producing bacteria. If there is no good coordination
of both clusters the result may be an accumulation of PenN due to its
ineffi cient conversion to cephalosporin C. Indeed, it is known since many
years ago that some improved
A. chrysogenum
strains (e.g. C10) accumulate
relatively large amounts of PenN, which is secreted to the broth, apparently
due to carbon catabolite regulation of the “late” gene cluster (Martín-Zanca
and Martin 1983). The evolutionary origin and the formation of the two
PenN/cephalosporin C clusters remain to be studied in detail.
As indicated in this chapter, several regulatory genes encoding global
transcriptional factors or regulators involved in differentiation (arthrospore
formation that is associated with high levels of cephalosporin biosynthesis
or with the formation of asexual conidiospores) are known. However, much
remains to the elucidated about the interaction of those transcriptional
factors with other proteins and with their target sequences in the promoters
of the
pcb
and
cef
genes. When the full genome sequence of
A. chrysogenum
becomes available, a signifi cant forward step in our understanding of global
regulation (systems biology) of this fungus will be achieved.
Finally, it is important to keep in mind the role of compartmentalization
in organelles of the enzymes involved in cephalosporin biosynthesis. We
only have a sketchy picture of the transport of intermediates (IPN and PenN)
into peroxisomes. The epimerization reaction converting IPN to PenN is
performed in peroxisomes and the late biosynthetic steps appear to be
performed in the cytosol. However, little is known about the transporters (or
alternative secretion mechanisms) involved in secretion of cephalosporin C
or the biosynthetic intermediates that are also secreted to the culture broth
(Martín et al. 2010).
Further molecular and cell biology studies of
A. chrysogenum
are really
needed to understand the important role of translocation of intermediates in
and out of the organelles and the mechanism of secretion of the fi nal product.
ACKNOWLEDGEMENTS
This work was supported by a grant of the European Union (Eurofung
QLRT-1999-00729 and Eurofungbase). Authors also wish to thank B.
Martín, J. Merino, A. Casenave and A. Mulero (INBIOTEC) for the excellent
technical assistance.
