Study on the specificity of DNA base sequence required for the transposition immunity of the (gamma) (delta) sequence

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A plasmid containing <gamma><delta> cannot acquire another copy of <gamma> <delta>by transposition (transposition immunity).We previously found that the 38-bp sequence of the <delta> terminal is sufficient to mediate the immunity. In the present study. however, the <gamma>-terminal 38 bp was shown to have weaker (1/45) immunity activity than the other. The <gamma>-terminal 38-bp sequence was then synthesized with and without a flanking 5-bp sequence and cloned into the SmaI-site of the plasmid pUC13 in order to see the effect of the flanking base sequence on the efficiency of transposition immunity. When the<gamma>-terminal 38 bp was cloned together with one of the natural flanking sequences GTAA. in stead of the artificial SmaI-site sequence TCCCC, immunity activity increased by 7-fold. It seems to suggest that the specificity of base sequence required for transposition immunity prefers AT-rich flanking sequence to GT-rich one. As for the base sequence between 36th and 38th, the substitution of TAT with ATG reduced the immunity by 1/28. Since the sequence ATG differs only one base from the sequnce AAG (the <delta>-terminal suquence), which had the strongest immunity, the specificity fo the base 37 must be extremely high.