Appl Environ Microbiol, February 1998, p. 771-774, Vol. 64, No. 2
0099-2240/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Differential Gene Expression in the Laccase Gene
Family from Basidiomycete I-62 (CECT 20197)
Mariana
Mansur,1
Teresa
Suárez,2 and
Aldo E.
González3,*
Departamento de Microbiologia Molecular,
Centro de Investigaciones Biológicas del Consejo Superior de
Investigaciones Cientificas, 28006 Madrid,3 and
Centro Nacional de Biotecnologia, Campus Universidad
Autónoma de Madrid, Cantoblanco, 28049 Madrid,2 Spain, and
Instituto Cubano de
Investigaciones de los Derivados de la Caña de Azúcar,
Havana, Cuba1
Received 18 August 1997/Accepted 14 November 1997
 |
ABSTRACT |
A family of genes encoding laccases has recently been described for
the basidiomycete I-62 (CECT 20197). Transcript levels of genes
lcc1, lcc2, and lcc3 were analyzed
under four different culture conditions to study their expression
patterns. Two of the laccase genes were clearly inducible by veratryl
alcohol: the lcc1 gene is inducible in early stages of
growth, and the lcc2 gene is also inducible but only when
the organism reaches the stationary phase. Transcript levels for the
third gene, lcc3, were uninduced by veratryl alcohol and
repressed by glucose.
 |
TEXT |
Lignin is a complex aromatic
biopolymer degradable by a few organisms, like white rot basidiomycetes
(7, 22). Industry has an increasing interest in
extracellular enzymes from white rot fungi, such as lignin and
manganese peroxidases and laccases, due to their potential to degrade
both highly toxic phenolic compounds and lignin (1, 8, 14, 29,
31). Elucidation of the catalytic mechanisms exerted by these
enzymes, characterization of the proteins, and cloning of the genes
encoding them have increased our understanding of the biochemistry and
genetics of this quite complex and unique extracellular oxidative
system (2, 19, 22, 35-38). The regulation of the expression
of genes belonging to families encoding lignin-degrading enzymes, such
as lignin peroxidase and manganese peroxidase isozymes produced by
Phanerochaete chrysosporium, has been reviewed by Broda et
al. (3, 4).
Originally, the numerous laccase isozymes were thought to be
posttranslational variants of the same gene product, but several groups
have been able to isolate and characterize several laccase genes and
cDNA copies (17, 18, 20, 23, 30, 32, 36-38). A total of
four different laccase cDNA sequences have been described for
Rhizoctonia solani (36), up to five laccase genes
have been described for Trametes villosa (37,
38), and three genomic DNA sequences coding for laccases have
been described for the basidiomycete I-62 (28), suggesting
that at least a part of the biochemical diversity of laccase isozymes
must be due to the genomic multiplicity of the laccase gene sequences.
Unfortunately, only a few reports up to now have studied the expression
of multiple genes encoding different laccase isozymes (10,
37). The basidiomycete I-62 efficiently degrades natural lignin
from beech wood, sugar cane bagasse, and wheat straw when cultured
under a variety of different physiological conditions (28a).
In this report, we demonstrate, by using Northern blot analysis, that
the expression of the three laccase genes previously identified and
cloned from the basidiomycete I-62 (28) is differentially
regulated. Two of these genes are sensitive to induction by veratryl
alcohol but at different stages of growth, while the third one clearly appears to be under catabolic repression.
Northern analysis of lcc genes.
Three genomic
sequences encoding laccases from the basidiomycete I-62 corresponding
to laccase genes lcc1, lcc2, and lcc3 have been cloned and sequenced (28). To study the expression pattern of each lcc gene, we chose a DNA fragment for each
gene that was able to hybridize exclusively with itself. The DNA
fragments used to specifically detect the transcripts of the genes
encoding the different laccases in the basidiomycete I-62 were a
0.25-kb ScaI-PstI DNA fragment from the
lcc1 gene, a 0.5-kb PstI-HindIII fragment from the lcc2 gene, and a 0.5-kb
KpnI-PstI fragment from the lcc3 gene.
The DNA fragments corresponded mainly to the 5' coding region,
where the different laccase genes are less conserved. A Southern
blot containing total DNA from the basidiomycete I-62 completely
digested with three restriction enzymes was hybridized, under
low-stringency conditions (28), with the internal 0.3-kb PstI-XhoI DNA fragment from the basidiomycete PM1
lac1 gene (9) (Fig.
1, lanes 1). Hybridizations of the same
blot with the three lcc-specific probes under highly
stringent conditions (33) gave a different pattern of bands
for each gene (Fig. 1, lanes 2 to 4). We could thus confirm that the
chosen probes were specific for each laccase gene and adequate to
perform transcription analysis. Cross hybridization among the selected
probes was below 5%. As can be observed, some bands revealed with the
heterologous probe (Fig. 1, lanes 1) do not correspond to any of the
three cloned genes (Fig. 1, lanes 2 to 4), suggesting the existence of
more than three laccase genes in the basidiomycete I-62 genome.

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FIG. 1.
Southern analyses of basidiomycete I-62 genomic DNA
digested with three restriction enzymes (PstI,
ScaI, and SmaI) and hybridized with a
heterologous probe from the basidiomycete PM1 lac1 laccase
gene under low-stringency hybridization conditions (lanes 1), an
lcc1-specific probe (lanes 2), an lcc2-specific
probe (lanes 3), and an lcc3-specific probe (lanes 4). MWS,
molecular sizes.
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To study the regulation of each lcc gene, I-62 was grown
under four different culture conditions. A defined culture medium previously described for I-62 (28) was used in this work; it includes 1% glucose as the carbon source, 1 mM ammonium tartrate as
the nitrogen source, and 4 mM veratryl alcohol as the inducer of
ligninolytic activities. Besides this inducing medium, the basidiomycete I-62 was grown in noninducing medium (1% glucose, 1 mM
ammonium tartrate), in fructose medium (1% fructose, 1 mM ammonium
tartrate, and 4 mM veratryl alcohol), and under nonlimiting nitrogen
conditions (1% glucose, 10 mM ammonium tartrate). All liquid cultures
were performed under agitation (100 rpm) at 28°C over 24 days. The
inoculum for the cultures was prepared as previously described
(28). The extracellular laccase activity has been measured
as described elsewhere (28) in the supernatants of basidiomycete I-62 cultures grown in these media and reached a maximal
level (3 U/ml) at day 8, followed by a decrease (up to day 12) and a
progressive increase in the late stationary phase, under induced
conditions (28).
Total RNA was extracted from mycelia harvested on different days during
the incubation period (We thank M. Espinosa, G. del Solar, and A. D. W. Dobson
for their critical reading of the manuscript.
M. Mansur acknowledges support from fellowships granted by the
Instituto de Cooperación Iberoamericano and Dirección
de Relaciones Internacionales del Consejo Superior de
Investigaciones Cientificas, Madrid, Spain. This work was supported by
grant BIO 93-0662-CO4-01 (CICYT, Madrid, Spain).
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