CLONING AND ANALYSIS OF THE GENES ENCODING THE TYPE-IIS RESTRICTION-MODIFICATION SYSTEM HPHI FROM HAEMOPHILUS-PARAHAEMOLYTICUS

The genomic region encoding the type IIS restriction-modification (R-M ) system Hphl (enzymes recognizing the asymmetric sequence 5'-GGTGA-3' /5'-TCACC-3') from Haemophilus parahaemolyticus were cloned into Esche richia coli and sequenced. Sequence analysis of the R-M Hphl system re vealed three adjacent genes aligned in the same orientation: a cytosin e 5 methyltransferase (gene hphIMC), an adenine N6 methyltransferase ( hphIMA) and the HphI restriction endonuclease (gene hphIR). Either met hyltransferase is capable of protecting plasmid DNA in vivo against th e action of the cognate restriction endonuclease. hphIMA methylation r enders plasmid DNA resistant to R.HindIII at overlapping sites. sugges ting that the adenine methyltransferase modifies the SI-terminal A res idue on the GGTGA strand. Strong homology was found between the N-term inal part of the m6A methyl-transferasease and an unidentified reading frame interrupted by an incomplete galE gene of Neisseria meningitidi s. The HphI R-M genes are flanked by a copy of a 56 bp direct nucleoti de repeat on each side. Similar sequences have also been identified in the non-coding regions of H. influenzae Rd DNA, Possible involvement of the repeat sequences in the mobility of the HphI R-M system is disc ussed.