Discovery of a ptsHl operon, which includes a third gene (ptsT), in the thermophile Bacillus stearothermophilus

摘要

The discovery of a ptsHl operon in Bacillus stearothermophilus XL-65-6 coupled with our previous report of a cel operon (Lai Ingram, J Bacteriol 175, 6441-6450, 1993) demonstrates that this thermophilic organism contains all of the genes required for cellobiose uptake by the phosphoenolpyruvate-dependent phosphotransferase system (PTS). Genes encoding the two general PTS proteins, HPr (ptsH) and enzyme l (ptsl), were cloned and sequenced. These form an operon which includes a third small gene (ptsT) of unknown function (encoded product Mr 18428). Both ptsH and ptsl were expressed at high levels from a single plasmid in Escherichia coli and complemented corresponding host mutations. Although the translated sequences for these genes were similar to homologues from Gram-positive mesophiles (64-77% identity), the B. stearothermophilus gene products were unusual in having a higher predicted pi and fewer negatively charged amino acid residues. Enzyme I also contained more alanine and leucine than mesophilic counterparts. Interestingly, ptsT inhibited the growth of E. coli ptsl mutants at 37°C. No such inhibition was observed during incubation at a lower temperature (30°C) or in E. coli DH5α, which is wild-type for ptsl. The predicted translation product from ptsT contained a high proportion of basic amino acids (27%) and had a high predicted pl (pH 117), properties similar to bacterial histone-like proteins, but did not exhibit homology to any sequences in the current database. Regions upstream and downstream from the ptsHl operon contain genes with homology to Bacillus subtilis ptsG and wapA (wall-associated protein), respectively.