DIFFERENT STRUCTURES OF SELECTED AND UNSELECTED ARAB-LACZ FUSIONS

Formation of araB-lacZ coding-sequence fusions is a key adaptive mutat ion system. Eighty-four independent araB-lacZ fusions were sequenced. All fusions carried rearranged MuR linker sequences between the araB a nd lacZ domains indicating that they arose from the standard intermedi ate of the well-characterized Mu DNA rearrangement process, the strand transfer complex (STC). Five non-standard araB-lacZ fusions isolated after indirect sib selection had novel structures containing back-to-b ack inverted MuR linkers. The observation that different isolation pro cedures gave rise to standard and non-standard fusions indicates that cellular physiology can influence late steps in the multi-step biochem ical sequence leading to araB-lacZ fusions. Each araB-lacZ fusion cont ained two novel DNA junctions. The MuR-lacZ junctions showed 'hot-spot ting' according to established rules for Mu target selection. The araB -MuR and MuR-MuR junctions all involved exchanges at regions of short sequence homology. More extensive homology between MuR and araB sequen ces indicates potential STC isomerization into a resolvable four-way s tructure analogous to a Holliday junction. These results highlight the molecular complexity of araB-lacZ fusion formation, which may be thou ght of as a multi-step cell biological process rather than a unitary b iochemical reaction.