STRESS RESPONSES - Cyanobacteria - page 663

wild-type cells grown at 30°C or 43°C. So also the same patterns of localization of these two proteins
have been found in DnaK3 mutant and in its suppressor. On the other hand, the localization of
DnaK3 protein in the membrane and membrane-bound fractions in the wild-type and its presence
exclusively in the cytosol in NBC001 mutant at high temperature has been noted. But in another
suppressor mutant NBC002, the pattern of localization of DnaK3 resembled the wild-type. A
correlation of these observations with D1 protein suggests that in the wild-type and dnaK3 mutant
at both 30°C and 43°C the full length D1 protein with molecular weight of 24 kDa was present (that
corresponded to the intermediate of D1 protein of Synechocystis sp. strain PCC 6803). The absence of
such an intermediate D1 protein in the suppressor mutant at either of the temperatures led them to
conclude that DnaK3 protein is involved in targeting the translational intermediates to the thylakoid
membrane. Using the lacZ gene as a reporter, Sato et al . (2007) demonstrated that dnaK2 is the only
gene that is up-regulated in S . elongatus PCC 7942 during heat stress. Cells exposed to high light or
high salt stress, dnaK2-lacZ expression was clearly inducible. On the other hand, dnaK1 and dnaK3
showed little response. In view of this, they postulated that DnaK2 might function as a general
molecular chaperone, e.g. in processes of protein folding, oligomer assembly, and stabilization of
the protein structure.
iii) The role of proteases : The key enzymes of the protein degradation machinery are designated as
peptidases or proteases. Peptidases catalyze the hydrolysis of peptides by digestion of the specifi c
bonds inside the target molecule. Peptidases can be divided into two large groups according to
their substrate specifi cities: endopeptidases and exopeptidases. Exopeptidases remove single or
several amino acid residues, dipeptides or tripeptides, from N- or C-termini, and accordingly can
be classifi ed into mono-, di- and tripeptidases, respectively (Kenny, 1999; Rawlings and Barret,
1999). Endopeptidases can also remove single or several amino acid residues, but in contrast to
exopeptidases, classifi cation of endopeptidases is based on the active proteolytic residues of the
enzymes, not on a substrate. Endopeptidases have been divided into four major groups: serine,
cysteine, aspartic and metallo-peptidases (Barrett, 1994, 1995; Callis, 1995; Kenny, 1999).
Various studies have shown that cyanobacteria possess a set of proteases belonging to families
such as Clp (Eriksson and Clarke, 1996; Porankiewicz et al. , 1998; Panichkin et al ., 2001), Deg
(Sokolenko et al ., 2002), FtsH (Mann et al ., 2000; Bailey et al ., 2001), Ctp (Shestakov et al ., 1994; Ivleva
et al ., 2002), Gsp (Zuther et al ., 1998) and SppA (Lensch et al ., 2001) that are also represented in non-
photosynthetic prokaryotes.
a) Clp proteases : There are four genes that encode Clp proteins ( ClpB , ClpC , ClpP and ClpX ) in
Synechocystis sp. strain PCC 6803. Of these, ClpC and ClpX genes exist as single copies and ClpB and
ClpP genes are present in multiple copies. Two Prochlorococcus strains (MED4 and MIT9313) also
possess similar copies of Clp homologues. In all these three organisms, ClpP is variable as ClpPI
appears to be monocistronic while ClpPII and ClpPIII are each part of bicistronic operons with ClpX
and ClpPIV , respectively (Mary et al ., 2004). S . elongatus PCC 7942 has 10 distinct Clp proteins. Of
these, four are Hsp100 chaperones (ClpB1-2, ClpC and ClpX), three Clp P proteins (ClpP1-3), ClpR
(which is ClpP-like protein) and two adaptor proteins (ClpS1-2) (Schelin et al ., 2002; Clarke et al .,
2005). ClpR, having a protein sequence similar to ClpP, seems to lack the catalytic triad (Ser-His-
Asp) characteristic of Ser-type proteases (Poranckiewicz et al ., 1999).
The cloning and sequencing of the single copy ClpB homologue from S. elongatus PCC 7942
was reported by Eriksson and Clarke (1996). ClpB gene consisted of 2,649 bp coding a polypeptide
of 883 amino acids. The absence of -10 mer and -35 mer sequences near the promoter region of
this gene signifi es that either constitutive σ 70 or the heat shock σ 32 factors do not have a role in