HETEROCYST - Cyanobacteria - page 252

heterocyst differentiation in Anabena sp. strain PCC 7120 after a nitrogen shift-down. Subsequently,
this has been confi rmed in case of N . muscorum and A . variabilis ATCC 29413 where the expression
of hupSL occurred about 24 h after nitrogen step-down and it has been suggested to be regulated at
the transcriptional level (Axelsson et al ., 1999; Happe et al ., 2000). A tsp is located 259 bp upstream of
hupS start codon of N . muscorum (Happe et al ., 2000; Lindberg et al ., 2000). A . variabilis ATCC 29413
lacks the 10.5 kb element interrupting hupL gene and the hupSL genes were transcribed as a single
operon. The presence of a tsp at 103 bp upstream of 5'-end of the transcript and a Fnr (fumarate
nitrate reductase regulator) binding site are the characteristic features of transcripts. The functional
nature of the HupSL has been assessed by the isolation of a hupSL disruptant mutant that could not
reoxidize the H 2 evolved during nitrogen fi xation, as a result of which the mutant evolved three
times more H 2 than the wild-type. A transcriptional analysis of hydrogenase genes in A . variabilis
ATCC 29413 revealed that the hupL gene transcripts appeared both in heterocysts and vegetative
cells and in vegetative cells of the ammonium grown organism (Boison et al ., 2000). Axelsson and
Lindblad (2002) found that the transcriptional activity of hupSL genes in case of N . muscorum and
N. punctiforme ATCC 29133 were enhanced in response to the addition of 9% H 2 in air that coincided
with enhanced uptake hydrogenase activity. A low level of oxygen and addition of nickel also
enhanced the hupSL transcript levels thus confi rming the earlier studies made on uptake hydrogenase
activity in case of Anabaena sp. strains CA and IF (Xiankong et al ., 1984), A . cylindrica (Houchins
and Burris, 1981b; Daday et al ., 1985), Oscillatoria subbrevis strain 111 (Kumar and Polasa, 1991) and
N. punctiforme ATCC 29133 (Oxelfelt et al ., 1995). In aerobically grown L . majuscula CCAP 1446/4
nitrogen fi xation with concomitant synthesis of HupL protein occurred in the dark phase. There
was an apparent HupL turnover with its degradation in the light phase and synthesis in the dark
phase emphasizing a correlation between nitrogen fi xation and uptake hydrogenase activity (Leitäo
et al ., 2005). The presence of putative NtcA-binding sites in the hupSL promoter of the heterocystous
cyanobacteria such as Nostoc PCC 7422 (Yoshino et al ., 2007), A . variabilis ATCC 29413 (427 bp upstream
of tsp of hupS; Weyman et al ., 2008) and in the non-heterocystous cyanobacteria L . majuscula CCP
1446/4 (in between 240 and 227 bp upstream of tsp of hupS; Leitão et al ., 2005) and Gloeothece sp.
ATCC 27152 (238 bp upstream of tsp of hupS ; Oliveira et al ., 2004) has been demonstrated. Holmqvist
et al . (2009) identifi ed the NtcA-binding site at -258.5 bp (upstream) to tsp in the promoter of hupSL
of N . punctiforme ATCC 29133. The removal of NtcA-binding site has no effect on the expression of
hupSL gene as truncated versions of the hupSL promoter without the NtcA-binding site as short as
-57 bp to tsp could elicit the expression of either gfp or luxAB as reporter genes. They suggested
that heterocyst-specifi c expression of the hupSL genes reside in this short promoter region or in
the downstream untranslated leader sequence. The hupSL genes with the probable location of the
maturation genes in relation to hyp -gene cluster in various fi lamentous heterocystous and non-
heterocystous cyanobacteria have been presented (Fig. 12).
Transcriptional analysis of hoxEFUYH genes, encoding the bidirectional hydrogenase, in Anabaena
sp. strain PCC 7120 showed that the transcripts appeared as two units because hoxEF and hoxUYH
are separated by a distance of 8.8 kb. However, along with hoxEF an ORF alr0750 also was transcribed
and similarly, hoxUYH transcript consisted of two other ORFs, alr0763 and alr0765 . Two tsps, one
occurring at 66 bp upstream of alr0750 and the other at 57 bp upstream of hoxU have been identifi ed.
Specifi c interactions between LexA and NtcA lead to a transcriptional regulation of hox genes and
the binding of these transcriptional regulators to the hox promoter region has been confi rmed by
electrophoretic mobility shift assays (Sjoholm et al ., 2007).