Biomedical Engineering Reference
In-Depth Information
In
A. chrysogenum
, initial studies reported that all enzymes of the
cephalosporin biosynthesis pathway might have a cytosolic location (Evers
et al. 2004, van de Kamp et al. 2009). Nevertheless, peroxisomal targeting
sequences have been found in CefD1 and CefD2 proteins showing that
the epimerization step probably takes place in peroxisomes (Martín et
al. 2010). In this compartmentalization of cephalosporin C biosynthesis,
CefP and CefM play a fundamental role in the substrate's entry (IPN) and
in product's exit (PenN) respectively, across the peroxisomal membrane.
Hereby, the role of CefP seems to be the transport of IPN from the cytosol
to the peroxisomal matrix where it is epimerized to form PenN. Next, the
epimerization product (PenN) is probably secreted from the peroxisomal
lumen to the cytosol by the CefM carrier.
Late Cluster: Genes for the Conversion of
Penicillin N to Cephalosporin C
The “late” cluster contains the
cefEF
and
cefG
genes
(Fig. 2).
These genes
are involved in the last two steps that drive the conversion of PenN
to cephalosporin C and are specifi c for cephalosporin C biosynthesis
(Gutiérrez et al. 1992).
The
cefEF
gene is 996 bp long and, as indicated above, it encodes a
bifunctional enzyme DAOC synthetase (expandase)/hydroxylase (332
amino acids with a deduced molecular mass of 36.4 kDa) (Samson et al.
1985, 1987).
The
cefG
gene is closely linked to the
cefEF
gene, but transcribed in the
opposite orientation (Fig. 2). Promoters of both
cefG
and
cefEF
genes are
contained in the intergenic region. Bidirectional promoters in fungi are
located in open chromatin regions and appear to be accessible to multiple
transcriptional factors that modulate, in a coordinate form, the expression
of the divergent genes. The
cefG
gene is 1479 nucleotides long, contains
two introns and encodes an acetyl-CoA: DAC acetyltransferase protein of
444 amino acids with an Mr of 49.2 kDa (Gutiérrez et al. 1992, Velasco et al.
1999). This enzyme catalyzes the fi nal step in cephalosporin C biosynthesis.
Regulatory Genes Controlling Cephalosporin Biosynthesis
and Differentiation of
Acremonium chrysogenum
The biosynthesis of secondary metabolites in fi lamentous fungi is often
associated with cell differentiation and development. In
A. chrysogenum
the relationship between sporulation and cephalosporin biosynthesis is
not yet known in detail. Three different cell types have been described
