Biology Reference
In-Depth Information
bp) and intergenic regions (one copy/1209 bp). The presence of one copy of HIP1 in the regions of
mobile introns signifi es that the HIP1 sequences are characteristic to the genomes of cyanobacteria. In
the genome of
N
.
punctiforme
ATCC 29133, multiple copies of HIP1 sequences are known to occur as
octanucleotide sequence (GCGATCGC). The frequency of such HIP1 is one sequence every 1200 bp
(Meeks
et al.
, 2001). Elhai
et al
. (2008) identifi ed mobile repeated elements known as small dispersed
repeats (SDRs) having 21 to 27 nucleotides and have been grouped into 8 families. Three of the
families represented by SDR4, SDR5 and SDR6 share a common predicted secondary structure. It
is signifi cant to note the distribution of SDR5 family within the octamer, HIP1.
11) Toxic and bioactive compounds
:
It is intriguing to fi nd out 62 ORFs encoding proteins involved
in the synthesis of microcystins in the genome of
N
.
punctiforme
ATCC 29133 (Meeks
et al
., 2001).
Microcystis
produces microcystins, cyanopeptolins and many other unidentifi ed bioactive substances. In
M
.
aeruginosa
NIES-84, the biosynthesis of microcystins is mediated by a non-ribosomal peptide
synthetase (NRPS) gene cluster (
mcyA
-
J
) and cyanopeptolins by cyanopeptolin synthase gene cluster
(
mcnA-C
and
mcnE-G
). A third novel unknown NRPS gene cluster is also located on the chromosome
of this organism. The organization of
mcy
gene cluster is similar to the other microcystin-producing
cyanobacteria in having two sub-clusters transcribed in opposite directions. The fi rst sub-cluster
encodes NRPSs with accessory domains (
mcyA
,
mcyB
and
mcyC
) and the second encodes polyketide
synthase (PKS) (
mcyD
) and mixed PKS-NRPS (
mcyE
and
mcyG
) and a racemase gene (
mcyF
). There
are three additional genes located downstream this sub-cluster, i.e. two genes involved in the
biosynthesis of microcystin (
mcyI
and
mcyJ
) and the third gene (
mcyH
) that is helpful in the transport
of microcystin. A thioesterase gene (
mcyT
) that is implicated in the induction of microcystin is
located within
mcy
-gene cluster in
Planktothrix agardhii
but this is located outside
mcy
gene cluster
in
M
.
aeruginosa
NIES-843 (Kaneko
et al
., 2007). Frangeul
et al
. (2008) compared the two strains of
M
.
aeruginosa
NIES-843 and PCC 7806 and confi rmed the existence of three large gene clusters for
NRPS/PKS. One of the clusters (PKSI) seems to govern the synthesis of aeruginoside and a protein
similar to enedyine-type antitumor antibiotic (characteristically produced in some actinobacteria).
The second (PKSII) cluster is responsible for the production of a number of tailoring enzymes and
a third PKS cluster synthesizes compounds of chalcone/Stilbene family. One of the three clusters is
suggested to govern the production of microcystins, cyanopeptolins and aeruginosins. The genes for
cyanopeptolin (
mcn
cluster) bear resemblance to the anabaenopeptolide genes of
Anabaena
strain 90
and to the cyanopeptolin genes of
M
.
wesenbergii
. However, additionally the genome of
M
.
aeruginosa
PCC 7806 revealed the presence of a gene cluster for the synthesis of patellamide, a substance that
is characteristically produced by the symbiotic cyanobacteria present in Ascidians. In contrast,
M
.
aeruginosa
NIES-843 produces microcyclamide that resembles patellamide. It is interesting to
note the presence of transporter genes for these secondary metabolites along with those that confer
resistance in the immediate vicinity of genes governing their synthesis. Thus the cells are able to
survive in presence of these substances when they are released to outside by the effl ux transporters
(those of the uncharacterized PKSI/PKSIII hybrid compound) present (Frangeul
et al
., 2008).
12) Nif genes
:
The nitrogen-fi xing non-heterocystous forms whose genomes have been sequenced
include
Cyanothece
sp. ATCC 51142,
C
.
watsonii
WH8501,
T
.
erythraeum
IMS101,
Synechococcus
RF-1 and
Synechococcus
sp. JA-3-3A,b. The heterocystous cyanobacteria whose genomes have been sequenced are
Anabaena
sp. strain PCC 7120,
A
.
variabilis
ATCC 29413,
N
.
punctiforme
ATCC 29133 and
No
.
spumigena
CCY9414. In all these organisms, all nitrogen fixation genes (
nif
genes) are located in a contiguous
cluster of 34 genes in a 28-kb region. Other structural genes in this region include
nifD
and
K
(that
encode dinitrogenase, Mo-Fe protein) and
nifH
(that encodes dinitrogenase reductase, Fe-protein). In