THE HOBO TRANSPOSABLE ELEMENT HAS TRANSPOSASE-DEPENDENT AND TRANSPOSASE-INDEPENDENT EXCISION ACTIVITY IN DROSOPHILID SPECIES

Mobility of the hobo transposable element was determined for several s trains of Drosophila melanogaster and several Drosophila species. Mobi lity was assessed by use of an in vivo transient assay in the soma of developing embryos, which monitored hobo excision from injected indica tor plasmids. Excision was detected in a D. melanogaster strain (cn; r y(42)) devoid of endogenous hobo elements only after co-injection of a helper plasmid containing functional hobo transposase under either he at shock or normal promoter regulation. Excision was also detected in D. melanogaster without helper in strains known to contain genomic cop ies of hobo. In Drosophila species confirmed not to contain hobo, hobo excision occurred at significant rates both in the presence and absen ce of co-injected helper plasmid. In four of the seven species tested, excision frequencies were two- to fivefold lower in the presence of p lasmid-borne hobo, hobo excision donor sites were sequenced in indicat or plasmids extracted from D. melanogaster ry(42) and D. virilis embry os. In the presence of hobo transposase, the predominant excision site s were identical in both species, having breakpoints at the hobo termi ni with an inverted duplication of proximal insertion site DNA. Howeve r, in the absence of hobo transposase in D. virilis, excision breakpoi nts were apparently random and occurred distal to the hobo termini. Th e data indicate that hobo is capable of functioning in the soma during embryogenesis, and that its mobility is unrestricted in drosophilids. Furthermore, drosophilids not containing hobo are able to mobilize ho bo, presumably by a hobo-related cross-mobilizing system. The cross-mo bilizing system in D. virilis is not functionally identical to hobo wi th respect to excision sequence specificity.