FIG. 4.

TBP overexpression stimulates rRNA production by polI. Strain RLY07 (ΔRRN5) was secondarily transformed with either an empty vector (pRS315), a vector expressing RRN5 (pRS315RRN5), or the TBP overexpression vector (pSH223). In parallel, strain RLY4811 (isogenic to RLY07 but also carrying a temperature-sensitive [ts] allele of RPA190) was transformed with the same three vectors. (A) At the permissive temperature (30°C) strains RLY07 and RLY4811 behave the same. They grow on galactose due to the presence of pNOY103 and fail to grow on glucose. Transformation with an expression vector for either RRN5 or TBP allows both strains to grow on glucose. (B) At the nonpermissive temperature (35.5°C) strain RLY4811 cannot grow on glucose even when RRN5 or TBP is expressed (right plate, bottom sectors). This result indicates that TBP overexpression stimulates rRNA production via polI and not via some other RNA polymerase.