STRESS RESPONSES - Cyanobacteria - page 667

during heat and light stresses. The essentiality of Deg proteases for PSII repair and D1 degradation
was re-examined by constructing a triple mutant ( htrA - hhoA - hhoB - ) using the wild-type strain of
Synechocystis . The wild-type, single and double mutants grew at 29°C or 37°C but the triple mutant
was unable to grow at high temperature. Thus Barker et al . (2006) demonstrated unambiguously
that although the Deg proteases are needed for photoprotection of Synechocystis , they do not play
essential role in D1 protein turn over and PSII repair. However, their role in protecting the extra-
cytoplasmic compartments from heat and light stress as well as from the oxidative damage caused
by the ROS generated by photosynthetic electron transport has been stressed (Florian et al ., 2005).
The properties of HhoA protease from Synechocystis sp. strain PCC 6803 have been studied and
the formation of hexameric complexes was shown to be regulated by the PDZ domain. Synechocystis
HhoA protein consisted of a single PDZ domain whose activity increased with a rise in temperature
and pH. The presence of Mg 2+ or Ca 2+ stimulated the proteolytic activity of HhoA (Huesgen et al .,
2007). Deg/HtrA proteases constitute important components of a network for PSII quality control in
chloroplasts and cyanobacteria. The functional role of Deg proteases in maintaining protein quality
control including PSII has been highlighted (Huesgen et al ., 2009).
c) FtsH proteases : This family is represented by bacterial FtsH protease that is a membrane integral
ATP-dependent peptidase (Tomoyasu et al ., 1993). It participates in various degradative processes
involving the degradation of translocase subunit SecY, heat stress transcription factor σ 32 and
transcriptional activator λCII (Herman et al ., 1995; Kihara et al ., 1995). It exists as a homodimeric or/
and tetrameric structure (Akiyama et al ., 1995) and this multimeric structure is required for the catalytic
activity (Karata et al ., 1999). Akiyama and Ito (2000) reported that the transmembrane organization is
essential for the proteolytic activity against the integral membrane proteins. Synechocystis sp. strain
PCC 6803 possesses four genes ( FtsH1 , FtsH2 , FtsH3 and FtsH4 ) that encode four respective FtsH
proteases. Gene inactivation studies revealed that two mutations (FtsH1 and FtsH3) were lethal
whereas the third (FtsH4) did not show any altered phenotype. The fourth mutation involving
FtsH2 caused altered pigmentation leading to impaired PSI biogenesis (Mann et al ., 2000). FtsH2 of
Synechocystis sp. strain PCC 6803, a homologue of Arabidopsis FtsH (Bailey et al ., 2001), is important
in controlling the degradation of D1 protein during its repair from photoinhibition (Zach et al ., 2005).
Mutants lacking FtsH2 ( slr0228 ) of Synechocystis showed impaired rate of degradation of D1 (Silva et
al ., 2003; Komenda et al ., 2006). It is not that the FtsH2 is involved in the removal of the breakdown
products of D1 but its function is envisaged at an early stage of D1 degradation (Haussuhl et al ., 2001).
D1 degradation is not completely blocked in mutants of Synechocystis that show extremely high rates
of D1 degradation (Komenda et al ., 2010). These evidences put together with those of Barker et al .
(2006) that selective turnover of D1 was still operational in triple mutants of Deg proteases, would
suggest the possibility of other FtsH proteases being associated with this process. An FtsH2 gene
construct with a glutathione S-transferase tag (GST-tagged) was introduced into Synechocystis sp.
strain PCC 6803 and when GST-tagged FtsH2 was purifi ed, FtsH3 was also copurifi ed along with it
(Barker et al ., 2008). In addition, preliminary negative stain electron microscopy revealed FtsH2 to
exist as hexameric complexes instead of being a dimer. So it means the Synechocystis FtsH proteases
can exist as hetero-oligomeric complexes, consisting of two types of FtsH subunits.
The involvement of FtsH and Deg proteases in the degradation of UV-B- damaged PSII reaction
center subunits D1 and D2 proteins of Synechocystis sp. strain PCC 6803 has been investigated. Gene
inactivation experiments showed that it is the FtsH not Deg proteases that are responsible for the
repair of PSII during and following UV-B irradiation. FtsH mutants (gene slr0228 ) exhibited increased