IS1236, A NEWLY DISCOVERED MEMBER OF THE IS3 FAMILY, EXHIBITS VARIED PATTERNS OF INSERTION INTO THE ACINETOBACTER-CALCOACETICUS CHROMOSOME

Analysis of spontaneous mutations in Acinetobacter calcoaceticus revea led a 1237 bp insertion sequence named IS1236 and possessing a nucleot ide sequence resembling those of members of the IS3 family. The chromo some of A. calcoaceticus strain ADP1 contains seven copies of IS1236 w hich appears to insert preferentially into pobR, the transcriptional a ctivator of the structural gene for p-hydroxybenzoate hydroxylase. IS1 236 creates tandem 3 bp DNA duplications flanking the sites of its ins ertion in pobR. Different duplication patterns are found following ins ertion of IS1236 into pcaH, a structural gene for protocatechuate 3,4- dioxygenase. Therefore the insertion properties of IS1236 appear to be influenced by its DNA target. Amino acid sequences associated with th e apparent transposase function have been conserved in ORFB of IS1236 whereas the presumed DNA-binding helix-turn-helix region of IS1236 ORF A exhibits substantial amino acid sequence divergence from its IS3 cou nterparts. IS1236 ORFA and ORFB coding sequences overlap considerably, and sequence evidence indicates mechanisms for ORFB expression in IS1 236 may resemble those employed by other members of the IS3 family. Po rtions of the IS1236 terminal repeats exhibit substantial sequence div ergence from other members of the IS3 family, but evolution appears to have conserved a mechanism preventing expression of the insertion seq uence genes as a consequence of transcriptional readthrough.