Biology Reference
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systems for the uptake of amino acids have been reported in a few cases. The kinetics of transport for
Glu in
A
.
cylindrica
(Rowell
et al
., 1977), Pro (Spence and Stewart, 1986) and Gln/Glu in
Anabaena
sp.
strain PCC 7120 (Flores and Muro-Pastor, 1988) have been investigated. Strasser and Falkner (1986)
reported the existence of a highly functional transport system for Glu/Asp uptake by a
Nostoc
sp., an
endosymbiont of
Geosiphon pyriforme
. High affi nity and low affi nity transport systems for Glu and Gln
have been reported by Chapman and Meeks (1983) in
A
.
variabilis
ATCC 29413. The
K
m
values for high
affi nity system were found to be 13.8 and 100 µM for Gln and Glu, respectively whereas the low affi nity
system showed
K
m
values to be 1.1 and 1.4 mM, respectively for Gln and Glu. A single transport system
for Leu (with a
K
m
of 10.8 µM) in the same organism was reported by Thiel (1988) that was inhibited
by Ala, Gly, Val and Met. Herrero and Flores (1990) showed that the uptake of L-Arg is governed by
an energy requiring high affi nity process and a low affi nity uptake system and the cells accumulated
the substrate in the former but not in the latter. Two such uptake systems have been noted for the
uptake of L-Lys also. Montesinos
et al
. (1995, 1997) characterized a number of amino acid analogue
resistant mutants of
Anabena
sp. strain PCC 7120. Mutants developed in presence of L-canavine
sulfate (50 µM), δ-hydroxylysine (50 µM), 4-aza-DL-leucine (100 µM), MSX (50 µM) and azaserine
(50 µM) were characterized for the transport of different amino acids, intracellular concentrations
and the type of amino acid released by them. On the basis of transport kinetics of various amino
acids they suggested that fi ve different types of transport systems are operative in
Anabaena
sp.
strain PCC 7120. These are: (i) a high affi nity system for basic amino acids (Arg and Lys; which
could transport His and Orn as well); (ii) two neutral amino acid transport systems N-I and N-II
(which could transport Ala, Asn, Gln, Met, Ser and Thr) and in addition the former could transport
Gly, Leu, Phe and Pro; (iii) a low affi nity, passive amino acid transport system for basic amino acids
and (iv) a low affi nity system for the uptake of acidic amino acids. In addition to
Anabaena
sp. strain
PCC 7120, Montesinos
et al
. (1997) included eight representative strains belonging to unicellular,
fi lamentous, heterocystous unbranched and branched members in their study for determining the
uptake of thirteen
14
C-labelled amino acids. The presence of two genes
natA
and
natB
that encode
a conserved component and a periplasmic binding protein, respectively of an ABC-type permease
has been demonstrated in all the cyanobacteria. Gene disruptant mutants for
natA
and
natB
of
Anabaena
sp. strain PCC 7120 released Ala, Val, Phe, Ile and Leu into the medium. Signifi cantly, the
diazotrophic growth potential as well as heterocyst differentiation was affected in these mutants.
The same group of workers further identifi ed all the subunits of N-I neutral amino acid permease.
These consist of two ATPases [encoded by
natA
(
all1046
) and
natE
(
all2912
)], two transmembrane
proteins [encoded by
natC
(
all1047
) and
natD
(
all1284
)] and a periplasmic binding protein [(encoded
by
natB
(
alr1834
)]. Two of these genes
natA
and
natC
are present as an operon. The expression of
natCA
operon and
natB
constitutively in the vegetative cells of
Anabaena
sp. strain PCC 7120 but not
in the heterocysts was supported by the corresponding increase in the transcript levels and also by
the use of P
natC-lacZ
fusion construct as a reporter gene. Ala was released at the highest level by
natB
disruptant mutants (CSS6) that exhibited 75% of the diazotrophic growth rate of the wild-type. The
growth rate of
natCA
mutants was reduced to 50% of that of the wild-type. The release of Ala was
affected in a mutant of
natB
that was unable to differentiate heterocysts. It means for highest release
of Ala the presence of functional heterocysts is required, meaning thereby that L-Ala is transported
into heterocysts from vegetative cells (Picossi
et al
., 2005). The component proteins of two ABC-
type amino acid transporters Bgt and N-II have been identifi ed in
Anabaena
sp. strain PCC 7120 by
Pernil
et al
. (2008). Bgt constitutes a high affi nity basic amino acid transporter with BgtA and BgtB as
components. BgtA is an ATPase subunit encoded by
alr4167
and BgtB is a composite protein that has
