Fig. 1. MPT5 negatively regulates HO expression. (A) Effect of the mpt5Δ mutation on expression of the HOp-lacZ reporter. Yeast strains K1107 (WT), TTC2 (mpt5Δ), TTC91 (ash1Δ) and TTC191 (mpt5Δ ash1Δ), harboring the HOp-lacZ reporter gene, were grown in YPD medium and assayed for β-galactosidase activity. Units shown are averages of five experiments. (B) Effect of MPT5 overexpression on the HOp-lacZ reporter. Yeast strains K1107 carrying HOp-lacZ transformed with vector YEp13 or YEpMPT5 were cultured in SC-Leu medium and assayed for β-galactosidase activity. Units shown are averages of three experiments for independent transformants. (C) The HO 3′-UTR is required for MPT5-mediated repression of HO. Yeast strains 10B (HOp-ADE2-HO 3′-UTR) and TTC47 (HOp-ADE2-ADH1 3′-UTR) were transformed with YEp13 or YEpMPT5. The resulting transformants were streaked on SC-Ade Leu (–Ade) plates and incubated for 3 days at 30°C.