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Sequences of four new heat-shock (HS) genes of Escherichia coli organized into
two operons were determined. The operon at 83 min specifies two proteins of 15.8
kDa (HslT) and 16.1 kDa (HslS), which are identical to IbpA and IbpB,
respectively. Expression of mRNA from a sigma 32-dependent promoter of the
hslTS/ibpAB operon is stimulated 30-75-fold upon temperature upshift. The
transcription start point (tsp) is located at a G, 96 bp upstream from the AUG
start codon of hslT/ibpA. The deduced amino acid sequences of HslT/IbpA and
HslS/IbpB are 48% identical to each other and were found to be remotely related
to the chloroplast low-molecular-weight HS protein, which is highly conserved
among plants. The second hs operon is much less actively stimulated by
temperature upshift, although it has a hs promoter that perfectly matches the
consensus of promoters recognized by sigma 32. Located at 88.9 min, the hslVU
operon specifies proteins of 19.1 kDa (HslV) and 49.6 kDa (HslU). Multiple tsp
were found in this operon. HslV is remotely related to the eukaryotic proteasome
proteins, and HslU is very similar to a Pasteurella haemolytica protein of
unknown function. Both HslU and the P. haemolytica protein share a
ATP/GTP-binding motif near their N-termini. The two operons described here are
transcribed counterclockwise on the standard genetic map.
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