Genomic Organization of a Diverse ACC Synthase Gene Family in Banana and Expression Characteristics of the Gene Member Involved in Ripening of Banana Fruits

The banana is one of the typical climacteric fruits with great economic importance in agriculture. To understand the basic mechanism underlying banana ripening, gene clones for banana ACC synthase (EC 4.4.1.14), a key regulatory enzyme in the ethylene biosynthetic pathway, were characterized. Genomic clones were analyzed by restriction mapping, and the data in conjunction with sequence comparisons with the previously isolated PCR fragments indicated that at least nine ACC synthase genes (MACS1-9) exist in the banana genome. Southern blot analysis showed they are located in different regions of the banana genome. Three lambda genomic clones (GMACS-1, -9, and -12) were completely sequenced, and gene structures of MACS1 (corresponding to alleles of GMACS-9 and -12) and MACS2 (corresponding to GMACS-1) were elucidated. The coding regions of these three genes were all interrupted by three introns. The size and location of introns are similar to the ACC synthase genes from tomato and Arabidopsis. Northern analysis showed that only MACS1 expressed during fruit ripening and was inducible by exogenous ethylene treatment, which indicates MACS1 is a significant member of the ACC synthase gene family related to ripening in banana fruit. The transcription initiation site of GMACS-12 containing MACS1 was defined. There is a TATTAAT sequence located at position -31 to -25 that qualifies as a TATA box. The delineation of transcription unit in MACS1 will facilitate the promoter studies for this gene and allow its specific functions involved in fruit ripening to be determined.