Phage 16-3 of the nitrogen fixing bacterium Rhizobium meliloti is a typical temperate phage as are the lambdoid phages of Escherichia coli. To date there is no genetic or physical evidence to indicate that 16-3 is related to the lambdoids. The genome of 16-3 is regulated by two repressors which bind to their operators1-9 (Fig. 1 a). Here we report that one of the 16-3 repressors (the C repressor) recognizes not only its own operator but also the operator of lambdoid phage 434, and that the 434 repressor reciprocally binds to the 16-3 operator. The 434 and 16-3 repressors show little sequence homology, except in the short operator recognition part, known as the helix-turn-helix motif10,11. The structure of the specific 434-repressor-DNA complex has been determined to 3.2 Å resolution by X-ray diffraction analysis 12. Three amino acid residues of the second α-helix of the helix-turn-helix motif make contacts with DNA base-pair edges in the DNA major groove12-14. Of these, two, Gin at position 1 of the α-helix and Gin at position 2, are conserved in 16-3 repressor. Furthermore, the putative 16-3 operator is homologous to the consensus structure of 434 operators15.
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