2 DOMAINS OF SUPERFAMILY-I HELICASES MAY EXIST AS SEPARATE PROTEINS

DNA and RNA helicases of superfamily I are characterized by seven cons erved motifs. The five N-terminal motifs are separated from the two C- terminal ones by a spacer that is highly variable in both sequence and length, suggesting the existence of two distinct domains. Using compu ter methods for protein sequence analysis, we show that PhoH, an ATP-b inding protein that is conserved in Escherichia coli and Mycobacterium leprae, is homologous to the putative N-terminal domain of the helica ses, whereas the putative E. coli protein YjhR is homologous to the C- terminal domain. These findings suggest that the N- and C-terminal dom ains of superfamily I helicases have distinct activities, with only th e N-terminal domain having the ATPase activity. It is speculated that PhoH and YjhR have evolved from helicases through deletion of the port ions of the helicase genes coding for the C- and N-terminal domain, re spectively.