Bt Research 2015, Vol.6, No.4, 1-12
3
Figure 1 Structure of thuringiensin I (Belder e Elderson. 2013)
(2005), for example, performed a screening of 575
strains of
B. cereus
and found 270 strains producers of
thuringiensins of type II.
2 Types of Thuringiensins
Levinson et al., (1990) described two types of
thuringiensins from assays of high-performance liquid
chromatography (HPLC). Thuringiensins of the type I
have low molecular weight, approximately 701Da,
and are composed of adenosine, glucose, a phosphate
group and gluconic diacid (Liu et al. 2010; Mac Innes
and Bouwer 2009). For many years it was believed
that thuringiensin I was a phosphorylated molecule
analogous to the adenine nucleotide with great structural
similarity to this nucleotide (Šebesta and Horska 1970;
Šebesta and Sternbach 1970; Šebesta et al., 1981).
However, recently, Liu et al., (2010) proposed that, in
fact, it is an oligosaccharide from adenine nucleoside.
Because of the similarity to adenine, the toxicity of
thuringiensin I was explained by the inhibition of
RNA polymerase biosynthesis, one of the key enzymes
in the transfer of genetic information. That's because
this exotoxin acts essentially in the step of the
polymerization of the polymerase reaction, competing
for binding sites with ATP (Devidas and Rehberger
1992; Perchat et al., 2005). More specifically,
thuringiensins I bind reversibly (without being
incorporated into the polymer) to a portion of adenosine
in the specific site of ATP (Šebesta and Sternbach
1970). The depression of RNA polymerase biosynthesis
was seen in assays with rats (Šebesta and Horska
1969), and as it is a fundamental process for all types
of life, thuringiensins I are toxic to almost all living
organisms (Belder and Elderson 2013).
Levinson et al., (1990), analyzing strains of
Bt
subsp.
thuringiensis
,
Bt
subsp.
tolworthi
and
Bt
subsp.
darmstadiensis
, reported that the gene that encodes the
thuringiensin I production is supported by plasmids.
Ozawa and Iwahana (1986) provided evidences that
the production of exotoxin is associated with a plasmid
of 62 Mdal in a strain of
Bt
subsp.
darmstadiensis,
but
that plasmids producers thuringiensins I are not
ubiquitous in
Bt
strains. Initially, it was believed that
an ABC transporter could be related to the secretion
and production of this exotoxin (Espinasse et al.,
2002a). The researches about thuringiensins are focused
on tests to measure their insecticidal activity and in
strategies for its detection and purification, because
studies on genetic determinants involved in its
biosynthesis are still scarce (Liu et al., 2014). Recently,
it was discovered that the
Thu3
gene, which is
homologous to ABC transporter is involved in the
secretion thuringiensin I. According to Liu et al.,
(2010) synthesizers gene of this exotoxin are encoded
by circular endogenous plasmids of 110 kb that
harboring
thu
cluster besides synthesizers genes of the
Cry1Ba proteins (Iatsenko et al., 2014). More specifically,
in the thuringiensin biosynthesis, genes
thuA
,
thuC
and
thuD
encode proteins responsible by the synthesis
of the key precursor of the toxin, a gluconic diacid
(precursor A) from glucose-6-phosphate. The
thuF
and
thu1
genes encode proteins involved in the
assemblage of thuringiensin I. The
thuE
gene encodes
the enzyme responsible for the synthesis and phospho-
rrylation of the toxin and the
thu3
gene encodes a
protein that acts in the release of thuringiensin I
mature, which may be secreted by the type IV-like
secretion system (T4SS) towards the cell (Liu et al.,
2010) (Liu et al., 2010).
Some studies report that the thuringiensin I production
is related to the presence of plasmids that also harbor
cry
and
vip1
/
vip2
genes (Espinasse et al. 2002b;
Cstagnola and Stock 2014; Iatsenko et al., 2014;
Levinson et al., 1990). (Espinasse et al., 2002a) and
(Perani et al., 1998), for example, report that the
production of high levels of thuringiensin I is linked to
the presence of plasmids carrying the gene encoding
the crystal protein Cry1b. However,
Bt
strains that do
not produce crystals may also produce thuringiensins I,
such as mutant strain of
B. thuringiensis
407-1 (Cry
-
),
which still synthesizes a pigment of soluble melanin
also secreted in the culture supernatant. Therefore, the
production of thuringiensin I can occur even in a
lineage that lost the plasmids containing the genes cry
(Espinasse et al., 2002b). It is believed that these
