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Scytonema sp. (Sinha et al ., 2001), Anabaena sp . (Rozema et al ., 2002; Sinha et al ., 2003) and Nodularia
sp . (Sinha et al ., 2003) has been reported. The synthesis of mycosporine-2-glycine (with maximum
absorption at 331 nm) was reported for the fi rst time in cultures of Euhalothece sp. (isolated from a
gypsum crust in a hypersaline saltern pond in Israel) when grown at highlight intensities (Kedar
et al ., 2002). A database on mycosporines and MAAs highlighting their molecular structures and
biosynthetic routes has been created (Sinha et al ., 2007). Shinorine synthesis in A . variabilis ATCC
7937 was not induced by heat stress either in presence or absence of UV-irradiation but its synthesis
in salt stress and ammonium treatment had synergistic effects with UV-irradiation. The induction
of shinorine synthesis was dependent on the salt and ammonium concentration (Singh et al ., 2008).
A combination of two genes for the biosynthesis of the common core (deoxygadusol) of all MAAs
existed only in case of A . variabilis ATCC 29413 and the other three cyanobacteria tested ( Anabaena
sp. strain PCC 7120, Synechocystis sp. strain PCC 6803 and S. elongatus PCC 6301) did not reveal gene
sequences governing the synthesis of MAAs.
vii) The role of scytonemin: The presence of scytonemin in the EPS of many cyanobacteria is
known. Scytonemin (544 molecular weight) is a dimeric molecule consisting of indolic and phenolic
subunits and its absorption is in between 325-425 nm with maximum at 370 nm. The 'scytonemin
skeleton' has been suggested to result by the condensation of tryptophan- and phenylpropanoid-
derived subunits (Proteau et al ., 1993). The synthesis of scytonemin in N . commune in response to
UV-B irradiation was minimal but it was inducible in high amounts by UV-A irradiation (350-400
nm; Ehling-Schulz et al ., 1997). Fleming and Castenholz (2007) investigated the role of scytonemin
synthesis in relation to UV-A irradiation and desiccation. In Chroococcidiopsis CCMEE 5056 and
N . punctiforme PCC 73102, the synthesis of scytonemin occurred in large amounts when periodically
desiccated (i.e. one day desiccated for every two days rehydrated) was irradiated with UV-A.
Scytonemin so synthesized in N . punctiforme PCC 73102 remained quite stable in the EPS despite
exposure to UV-A for two months. In Chroococcidiopsis CCMEE 246 that produces scytonemin
constitutively in visible light, periodic desiccation partially inhibited scytonemin production.
(viii) Transcriptional regulation : Suzuki et al . (2004) identifi ed two ORFs, alr0295 and alr2325
that encode proteins similar to cAMP receptor proteins (CRP) in Anabaena sp. strain PCC 7120
and designated these genes ancrpA and ancrpB . The corresponding proteins AnCrpA and AnCrpB
showed binding to cAMP, the former was more specifi c to cAMP than the latter that showed weak
binding to cAMP as well as cGMP. AnCrpA possesses a core of three amino acids (Arg-180, Glu-
181 and Arg-185) as DNA-binding region specifi c to the fi ve base pair sequence 5'-TGTGA-3' of
DNA just as in case of E . coli . AnCrpB lacks this DNA-binding motif and its C-terminal sequence
does not bear any resemblance to E . coli CRP, SYCRP1 ( Synechocystis cAMP receptor protein) and
AnCrpA. The inability of AnCrpB to bind to DNA is due to difference in the sequence of amino
acids at the DNA binding motif (Val-187, Glu-188 and Lys-192) of which Val-187 could not hydrogen
bond with DNA. It is suggested that AnCrpB acts as a transcriptional regulator. AnCrpA has been
suggested to regulate nif -related gene expression in Anabaena sp. strain PCC 7120 when grown on
nitrate. The ancrpA disruptant mutant exhibited down-regulation of several genes of nitrogen fi xation
when grown on nitrate. It was possible to identify the specifi c genes by oligonucleotide microarray
and qRT-PCR. The binding of AnCrp to 5'-upstream region of nifB gene was confi rmed in presence
of cAMP using electrophoretic mobility shift assays (Suzuki et al ., 2007). Higo et al . (2007) conducted
a global transcriptional analysis in response to rehydration in Anabaena sp. strain PCC 7120. During
dehydration, as many as 133 genes were down-regulated and 29 genes were up-regulated. The process
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