SummaryThe universally distributed heat‐shock proteins (HSPs) are divided into classes based on molecular weight and sequence conservation. The members of at least two of these classes, the HSP60s and the HSP70S, have chaperone activity. Most HSP60s and many HSP70s feature a striking motif at or near the carboxyl terminus which consists of a string of repeated glycine and methionine residues. We have altered thegroELgene (encoding the essentialEscherichia coliHSP60 chaperonin) so that the protein produced lacks its 16 final (including ninegly, and fivemet) residues. This truncated product behaves like the intact protein in severalin vitrotests, the only discernible difference between the two proteins being in the rate at which ATP is hydrolysed. GroELtrcan substitute for GroELin vivoalthough cells dependent for survival on the truncated protein survive slightly less well during the stationary phase of growth. Elevated levels of the wild‐type protein can suppress a number of temperature‐sensitive mutations; the truncated protein lacks this ab