FIG. 7.

Nitrogen starvation activates Ty1 transcription and transposition. (A) β-Galactosidase activity determined in diploid cells obtained by crossing the FYBL1-23D derivatives carrying different TY1A-lacZ fusions with L5684. Cells were either grown in SD medium (High N, high nitrogen) or in SLAD medium (Low N, low nitrogen). Details on lacZ fusions are given in the legends to Fig. 4 and 5. (B) Retrotransposition of the endogenous Ty1his3AI-270 element. Patches correspond to four different diploid strains obtained by crossing JC297 with LV148. SD plates are shown after 4 days of incubation at 30°C. On the left are shown diploid cells replica plated onto the SD plate after growth on SLAD plus histidine (nitrogen-starved diploid cells), and on the right are shown diploid cells replica plated onto an SD plate after growth on SD plus histidine (nitrogen-fed diploid cells). The nonrandom localization of His⁺ colonies probably results from higher growth at the edge of the patches.