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J. Bacteriol., Aug 1996, 4839-4846, Vol 178, No. 16
MJ Eriksson and AK Clarke
The heat shock protein CIpB (HSP100) is a member of the diverse group of
Clp polypeptides that function as molecular chaperones and/or regulators of
energy-dependent proteolysis. A single-copy gene coding for a ClpB homolog
was cloned and sequenced from the unicellular cyanobacterium Synechococcus
sp. strain PCC 7942. The predicted polypeptide sequence was most similar to
sequences of cytosolic ClpB from bacteria and higher plants (i.e., 70 to
75%). Inactivation of clpB in Synechococcus sp. strain PCC 7942 resulted in
no significant differences from the wild-type phenotype under optimal
growth conditions. In the wild type, two forms of ClpB were induced during
temperature shifts from 37 to 47.5 or 50 degrees C, one of 92 kDa, which
matched the predicted size, and another smaller protein of 78 kDa. Both
proteins were absent in the delta clpB strain. The level of induction of
the two ClpB forms in the wild type increased with increasingly higher
temperatures, while the level of the constitutive ClpC protein remained
unchanged. In the delta clpB strain, however, the ClpC content almost
doubled during the heating period, presumably to compensate for the loss of
ClpB activity. Photosynthetic measurements at 47.5 and 50 degrees C showed
that the null mutant was no more susceptible to thermal inactivation than
the wild type. Using photosynthesis as a metabolic indicator, an assay was
developed for Synechococcus spp. to determine the importance of ClpB for
acquired thermotolerance. Complete inactivation of photosynthetic oxygen
evolution occurred in both the wild type and the delta clpB strain when
they were shifted from 37 directly to 55 degrees C for 10 min. By
preexposing the cells at 50 degrees C for 1.5 h, however, a significant
level of photosynthesis was retained in the wild type but not in the mutant
after the treatment at 55 degrees C for 10 min. Cell survival
determinations confirmed that the loss of ClpB synthesis caused a fivefold
reduction in the ability of Synechococcus cells to develop thermotolerance.
These results clearly show that induction of ClpB at high temperatures is
vital for sustained thermotolerance in Synechococcus spp., the first such
example for either a photosynthetic or a prokaryotic organism.
Copyright © 1996, American Society for Microbiology
The heat shock protein ClpB mediates the development of thermotolerance in the cyanobacterium Synechococcus sp. strain PCC 7942
Department of Plant Physiology, University of Umea, Sweden
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