Fig. 2. Transcriptional regulation of pub1⁺ and pub2⁺. (A) Northern analysis of pub1⁺ and pub2⁺ mRNA in mitotically growing cells. Synchronous cultures were attained by using the cdc25-22 temperature-sensitive mutant K164-9 (see the Materials and Methods). A 2.3 kb fragment containing pub1 and a 2.0 kb fragment containing pub2 were used as hybridization probes. The cdc22⁺ mRNA was also traced as an example whose levels fluctuate during cell cycle, peaking at the G1/S boundary (Hofmann et al., 1994). A 2.7 kb fragment containing cdc22 was used as a probe. The septation index, which reaches a peak in G1/S phase, was determined in order to monitor progression of the cell cycle. The agarose gels were stained with ethidium bromide, and ribosomal RNAs were used as loading controls and size markers. (B) Northern analysis of pub1⁺ and pub2⁺ mRNA after nitrogen starvation. h⁹⁰ wild-type (L968) and ste11 (KJ33-1A) strains were cultured in EMM2-N liquid sporulation medium. The same probes as in (A) were used.

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