HETEROCYST - Cyanobacteria - page 250

Genome sequencing of a number of cyanobacteria has facilitated the recognition of genes
governing the synthesis of hydrogenases. The hydrogen uptake ( hup ) genes encode the uptake
hydrogenases while hydrogen oxidation ( hox ) genes encode the bidirectional hydrogenases. Post-
translational steps lead to maturation of uptake and bidirectional hydrogenases. These involve
the insertion of the metal ions into the active site(s) of hydrogenases and attachment of ligands to
the Fe-atom. A set of six genes, hypABCDEF ( hyp for hydrogenase pleiotropic) encode the proteins
necessary for this function. The hyp -gene cluster of Anabaena sp. strain PCC 7120 is depicted in
Fig. 11A. A fi nal step in the maturation process involves the proteolytic cleavage of approximately
30 amino acid residues at the C-terminal end of the large subunit of the two hydrogenases mediated
by hupW (for uptake hydrogenase) and hoxW (for bidirectional hydrogenase) genes, the products of
which are known as C-terminal endopeptidases. In S. elongatus PCC 7942, Synechococcus sp. (strains
WH8102, CC 9902, CC 9905), Gloeobacter violaceus PCC 7421 and strains of P . marinus (MIT 9312, MIT
9313, CCMP 1378, NATL 2A, sub. sp. marinus CCMP 1375) the hup , hox and hyp genes are absent.
Certain other strains of Synechococcus sp. (PCC 7942, PCC 6308, PCC 6301 and PCC 7002) possess hox
and hyp genes while hox genes alone are represented in Synechocystis sp. strain PCC 6803, A . platensis ,
Spirulina subsala and Prochlorothrix hollandica . In the heterocystous forms N . punctiforme and Nostoc
Mitsui 38901 are exception in having hup and hyp genes whereas in other heterocystous members
( A . variabilis ATCC 29413, Anabaena sp. strain PCC 7120, Anabaena siamensis TISTR 8012) hup , hox
and hyp genes are represented. In Lyngbya majuscula , a non-heterocystous cyanobacterium that fi xes
nitrogen under microaerophilic conditions, the hup , hox and hyp genes are present (Tamagnini et
al ., 2002, 2005, 2007).
Uptake hydrogenases of cyanobacteria are heterodimeric proteins consisting of a large subunit
(α subunit) encoded by hupL and a small subunit (β subunit) encoded by hupS . HupL (60 kDa)
possesses the active site with four cysteine residues two of which join the Fe and Ni atoms. HupS
(35 kDa) contains the Fe-S clusters, important in passing electrons from the active site to the electron
acceptor in the respiratory chain to produce ATP. The bidirectional hydrogenase of cyanobacteria
is encoded by hoxEFUYH genes. The properties of the bidirectional enzyme have been understood
from several cyanobacterial species (Hallenbeck and Benemann, 1978; Llama et al ., 1979; Asada et al .,
1987; Schmitz et al ., 1995, 2002). The bidirectional enzyme is sensitive to O 2 , thermotolerant and has
high affi nity to H 2 . Unlike the uptake hydrogenase, the bidirectional hydrogenase does not require
the biosynthesis of nitrogenase as a pre-requisite for its biosynthesis as has been shown in some
unicellular strains (Howarth and Codd, 1985). Schmitz et al . (2002) showed that the enzyme from
Synechocystis sp. strain PCC 6803 consists of a large subunit HoxH with the active site consisting
of six cysteine residues involved in the binding of nickel. The small subunit HoxY contains four
cysteine residues that are supposedly involved in coordinating the putative [4Fe-4S] cluster.
HoxHY together constitute the hydrogenase component while HoxEFU (encoded by hoxEFU ) is
the diaphorase component (dihydrolipoamide:NAD oxidoreductase) that transfers electrons to
NAD(P) + . The HoxU and HoxF proteins are the small and large subunits of the diaphorase moiety.
The HoxEFUYH subunits are of 20, 61, 28, 24 and 49 kDa molecular weight, respectively with the
dimeric enzyme complex of bidirectional hydrogenase Hox (EFUYH) 2 , attaining a molecular weight
of 375 kDa. Comparison of gene sequences of hoxEFUYH or the deduced amino acid sequences of
the corresponding proteins of A . variabilis ATCC 29413, A . variabilis IAM M58, Anabaena sp. strain
PCC 7120, Synechococcus sp. strain PCC 6301 and Synechocystis sp. strain PCC 6803 revealed a high
degree of homology with the identity in case of A . variabilis ATCC 29413 and Anabaena sp. strain
PCC 7120 reaching as high as 95% (Tamagnini et al ., 2002, 2007).