Biology Reference
In-Depth Information
their autodegradation
in vitro
was absent; and (v) lastly, according to Risser and Callahan (2007)
we are presently not in a position to comprehend the intricate relationship between HetR protein
turn over and protease activity on the one hand and protease activity of HetR and heterocyst
differentiation on the other.
iii) hetP
:
Mutation in P6 of
Anabaena
sp. strain PCC 7120 caused a Het
-
phenotype (Ernst
et al
., 1992).
Cloning and sequencing of the corresponding wild-type ORF located at approximately 3.44 Mb on
the physical map of the chromosome revealed it to be an ORF of 474 or 525 bp encoding a protein
of 159- or 175- amino acid residues, respectively that has no match in the databases. Fernandez-
Pinas
et al.
(1994) named this ORF in P6 as
hetP
. Although reconstruction of the mutation has been
successful, the wild-type
hetP
sequence could not complement the mutant until it is associated with
an additional ORF situated at the 3'-end of
hetP
. It could not be conclusively ascertained whether
the Het
-
phenotype of P6 has been caused due to loss of
hetP
as such or due to a polar effect of
the Tn5 insertion or both. When
luxAB
has been used as the reporter gene to study transcription,
mRNA of
hetP
accumulated 2.5 fold higher in a double recombinant within 6 h when compared
to 12 h taken by the single recombinant. However, transformation of P6 or wild-type strain with a
plasmid (pDU1) bearing
hetP
caused Mch-phenotype in both the transformants. In this respect, the
effect of ectopic expression of
hetP
resembled the effects of such an expression of
hetR
, the master
regulator of heterocyst differentiation. Indeed, Higa and Callahan (2010) demonstrated that the
expression of
hetP
on a replicative plasmid could even bypass the requirement for
hetR
, when a
hetR
deletion mutant was transformed with a plasmid bearing
hetP
gene sequence. Such a transformant
differentiated heterocysts that appeared normal but possessed nitrogenase activity under anaerobic
conditions despite the production of Hgls and envelope polysaccharides. A
hetP
deletion mutant
differentiated very few heterocysts and they further showed that there are two homologues of
hetP
represented by ORFs
asl1930
and
alr2902
that could readily complement the
hetP
deletion mutant.
This is in sharp contrast to the failure of wild-type
hetP
gene sequence to complement
hetP
disruptant
mutant (Fernandez-Pinas
et al
., 1994). The use of
hetP
-
gfp
as a reporter gene in strains lacking HetR,
PatA or HetF showed GFP fl uorescence typical of HetR fl uorescence after a nitrogen shift-down.
The presence of fi ve tsps located at -727, -545, -208, -177 and -12 bp in the promoter region of
hetP
and a probable HetR-binding region of a seven base pair invert repeat between -727 and-545 tsps
confi rmed the regulation of
hetP
by HetR. Haselkorn (2010) examined the role of
hetP
as a new player
in the cascade controlling heterocyst differentiation mediated by HetR.
iv) hetC
:
Khudyakov and Wolk (1997) identifi ed
hetC
gene while characterizing the transposon Tn5-
1058 induced mutant C3 of
Anabaena
sp. strain PCC 7120 that did not differentiate heterocysts after a
nitrogen step-down. The transposition of Tn5-1058 in an ORF designated by them as
hetC
encodes a
protein of 1,044 amino acid residues. It is located 1,165 bp 5'-end of
hetP
. The HetC protein exhibited
a similarity to Hly family of bacterial protein exporters in its two-thirds C-terminal sequence while
the rest of the N-terminal portion has no similarity to any protein in the database. Functional copies
of
hetC
supplied in trans position in C3 mutant on a multicopy plasmid complemented
hetC
and
restored the Fox
+
and Het
+
phenotype of the wild-type. The participation of
hetC
in the regulation
of heterocyst differentiation has been confi rmed by its expression during the fi rst 4 h after nitrogen
step-down. The presence of a canonical NtcA-binding sequence (GTAACATGAGATAC) upstream
of the tsp located at -571 bp in the promoter region of
hetC
lends support to the suggestion that
hetC
is another gene that is activated by NtcA (Muro-Pastor
et al
., 1999). Further, in an
ntcA
interrupted
mutant (CSE2) the expression of
hetC
has been impaired. In a
hetC
disruptant mutant (DR1653),
the induction of
hetR
by NtcA is not disturbed and is similar to wild-type cells indicating that
hetR
