ClpB in a cyanobacterium: predicted structure, phylogenetic relationships, and regulation by light and temperature.

The sequence of a genomic clone encoding a 100-kDa stress protein of Plectonema boryanum (p-ClpB) was determined. The predicted polypeptide contains two putative ATPase regions located within two highly conserved domains (N1 and N2), a spacer region that likely forms a coiled-coil domain, and a highly conserved consensus CK2 phosphorylation domain. The coiled-coil region and the putative site of phosphorylation are not unique to p-ClpB; they are present in all ClpB sequences examined and are absent from the ClpB paralogs ClpA, ClpC, ClpX, and ClpY. Small quantities of a 4.5-kb p-clpB transcript and 110-kDa cytosolic p-ClpB protein were detected in cells grown under optimal conditions; however, increases in the quantities of the transcript and protein were observed in cells grown under excess light and low temperature conditions. Finally, we analyzed ClpA, ClpB, and ClpC sequences from 27 organisms in order to predict phylogenetic relationships among the homologs. We have used this information, along with an identity alignment, to redefine the Clp subfamilies.