The operon from and other Gram-positive bacteria with low G+C DNA content contains additional heat-shock genes, including . The gene encodes a transcription factor that negatively regulates heatshock genes and is uniformly present in all Gram-positive bacteria studied to date. An homologue is also present in species, and , organisms that diverged early on from the common ancestor of all Gram-positive bacteria and , according to 16S rRNA phylogeny. A partial, protein-based phylogenetic tree, derived using amino acid sequence homology of proteins from Gram-positive bacteria, is presented here, and the results are compared with the phylogenetic trees generated from 16S rRNA, and sequences. The location of the gene and the genome organization of the operon support the division of all Gram-positive bacteria into three major groups: one group contains high-G+C Gram-positive bacteria, and two others contain low-G+C Gram-positive bacteria. Among the Gram-positive bacteria with low G+C DNA content, the results indicate that there is a close phylogenetic relationship between species and species on the one hand and between and on the other. and species also exhibited a close relationship. A hierarchical arrangement of Gram-positive bacteria based on HrcA sequences is proposed as an additional refinement of the phylogenetic relationships within this important bacterial group.


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