A phylogenetic tree constructed by 5S rRNA analysis is composed of three major branches in eubacteria: high G + C Gram+, low G + C Gram+ and Gram- [Hori and Osawa, Mol. Biol. Evol. 4 (1987) 445-472]. We have shown that the characteristic dnaA region is common among Escherichia coli (Gram-), Pseudomonas putida (Gram-), and Bacillus subtilis (low G + C Gram+). We have now determined the structure of the dnaA region of Micrococcus luteus, as a representative of the last branch, high G + C Gram+. The dnaA gene and at least three other genes, rnpA, rpmH and dnaN were found to be conserved in M. luteus. Large nontranslatable regions were found flanking the dnaA gene. The upstream region is conserved in the four bacteria so far examined. On the other hand, the downstream region is conserved only in Gram+ bacteria, M. luteus and B. subtilis. The consensus sequence of the DnaA box in M. luteus seems to be TTGTCCACA, in contrast to TTATCCACA of other bacteria. These results confirm our hypothesis that the dnaA region is the replication origin of the ancestral bacteria and that the essential feature of the DnaA protein and DnaA-box combination is conserved in eubacteria.