Expression cloning in Fe2+ transport defective yeast of a novel maize MYC transcription factor

A complementation approach of the yeast fet3fet4 mutant strain, defective i n both low- and high-affinity iron transport, was initiated as an attempt t o characterize the Fe(III)-mugineic acid (MA) transporter from grasses. A m aize cDNA encoding a novel MYC transcription factor, named 7E, was cloned b y screening an iron-deficient maize root cDNA expression library on a minim um media containing Fe(III)-deoxyMA as a unique iron source. 7E expression restored growth specifically to the fet3 fet4 mutant strain. It did not aff ect growth rate of a trk1 trk2 potassium transport defective yeast strain o r parental W303 strain growth rate, No Fe-55 uptake increase was observed i n 7E transformed fet3 fet4 yeast during short-term kinetics. However, the i ron accumulation in these cells was 1.3-fold higher than in untransformed c ells after a 24-h period. The 7E protein contained 694 amino acids and had a predicted molecular mass of 74.2 kDa. It had 44% identity with the RAP-1 protein, a 67.9-kDa MYC-like protein from Ambidopsis thaliana which binds t he G-box sequence via a basic region helix-loop-helix (bHLH), without requi ring heterodimerization with MYB proteins. Phylogenic comparisons revealed that the maize 7E protein was related to the Arabidopsis thaliana RAP-1 pro tein and to the Phaseolus vulgaris PGI. This similarity was particularly ev ident for the bHLH domain, which was 95% identical between maize 7E and Ara bidopsis thaliana RAP-1. 7E, RAP-I and PG-I proteins revealed a plant MYC-l ike sub-family that was more related to the maize repressor-like IN1 than t o maize R proteins. 7E mRNA was detected in both roots and leaves by the No rthern analysis. The amount of 7E mRNA increased, in response to iron starv ation, by 20 and 40% in roots and leaves, respectively. The relationship be tween iron metabolism and myc expression in animal cells is discussed. (C) 1998 Elsevier Science B.V. All rights reserved.