Biology Reference
In-Depth Information
trehalose synthesis pathways were found (e.g.
Wolf, Krämer, & Morbach,
2003
), such as TreYZ. In this pathway, a polysaccharide (i.e. glycogen) pre-
cursor is first changed at the terminal end to α 1,1 sugar bound by maltool-
igosyltrehalose synthase (TreY), and then the trehalose is cleaved off by
maltooligosyltrehalose trehalohydrolase (TreZ):
(TreY) α-1,4-polyglucose → α-1,1-maltooligosyltrehalose
(TreZ) α-1,1-maltooligosyltrehalose → trehalose + α-1,4 polyglucose (n-2)
Molecular analyses of cyanobacterial trehalose synthesis revealed that the
TreYZ pathway seems to be mostly used. The occurrence of the TreYZ
pathway among cyanobacteria was first verified in
Nostoc
7120 during the
study of a gene cluster, which was previously found to be upregulated under
desiccation (
Higo, Katoh, Ohmori, Ikeuchi, & Ohmori, 2006
). In addition
to genes for TreY (
all
0167), TreZ (
all
0168), this operon codes for a trehalase
(TreA,
all
0166), which is able to hydrolyse trehalose. It should be mentioned
that
Nostoc
7120 accumulates only sucrose under salt stress, while trehalose
seems to be made in response to drought stress. Subsequently, the TreYZ
pathway has also been found in other trehalose-accumulating cyanobac-
teria, such as
Nostoc punctiforme
IAM M-15 (
Yoshida & Sakamoto, 2009
),
Arthrospira platensis
NIES-39 (
Ohmori, Ehira, Kimura, & Ohmori, 2009
),
and
Nostoc flagelliforme
(
Wu, He, Shen, Zhang, & Wang, 2010
). Using the
TreYZA proteins from
Nostoc
7120 in Blast searches, genes showing high
similarities were found in 19 cyanobacterial genomes (
Table 2.1
). These
strains all belong to the group of beta-cyanobacteria; many of them are
filamentous and/or N
2
-fixing cyanobacteria, which display often a high-
desiccation tolerance (e.g.
Microcoleus vaginatus
FGP-2;
Starkenburg et al.,
2011
). However, genomes of a few unicellular, non-N
2
-fixing strains con-
tain also these genes. Interestingly, the genomes of the two thermophilic
Synechococcus
strains ( JA-3-3Aba) from Yellowstone National Park, USA
contain TreYZ-coding genes. Most probably, these strains synthesize treha-
lose as thermoprotectant in their hot environment because this disaccharide
exhibits excellent stabilizing activity for macromolecules at high tempera-
tures (
Furuki et al., 2009
).
With the only exception
C. watsonii
strain WH 8501, genes coding the
E. coli
-like OtsAB pathway for trehalose synthesis are absent from cyano-
bacteria (
Table 2.1
). Only the genome of this unicellular, marine N
2
-fixing
cyanobacterium codes for a large fusion protein comprising OtsA and OtsB
