β-半乳糖苷酶(β-galactosidase)通过分解细胞壁半纤维素切除半乳糖键而参与果实软化.为了阐明香蕉(Musa sp.)果实成熟过程中的软化与细胞壁代谢酶β-半乳糖苷酶基因表达之间的关系,采用RT-PCR方法,从成熟香蕉果实果肉中分离了编码β-半乳糖苷酶基因的部分cDNA(MA-Gal),序列分析表明,MA-Gal包含927 bp,编码309个氨基酸,包含5个β-半乳糖苷酶结构域(典型真核生物中β-半乳糖苷酶包含7个结构域),推导的MA-Gal蛋白质中有β-半乳糖苷酶蛋白的催化活性部位GGPIILSQIENEY(F);系统进化树分析结果表明MA-Gal属于第一类β-半乳糖苷酶基因(该类主要在果实中表达);β-半乳糖苷酶活性和硬度的变化表明其与香蕉果实硬度变化密切相关;Northern分析显示,跃变前期的果肉中,MA-Gal基因的表达量很低,后随着果实的软化表达量不断增加,并在呼吸跃变后达到最高.所有结果表明,MA-Gal参与香蕉果实成熟过程中的软化.%β-Galactosidase is thought to be involved in fruit softening through cleaving β (1 →4) galactan bonds in cell wall hemicellulose. To study the relationship between fruit softening and β-Gal during banana (Musa sp.) fruit ripening, a β-Gal cDNA fragment, named MAGal, has been cloned from banana fruit pulp using RTPCR in this study. The results of sequence analysis showed that MA-Gal contained 927 bp, encoding a polypeptide of 309 amino acids, the deduced protein was highly homologous to plant β-galactosidase expressed in fruit ripening. The MA-Gal putative amino acids have five homologous domains (typical eukaryotic β-Gals have seven domains), contain the GGPIILSQIENEY(F) motif, which is a presumptive catalytic active site conserved in all β-Gals. Phylogenetic analysis showed that MA-Gal was in plant β-Gal group Ⅰ, which was expressed in fruit tissue. The changes in β-Gal activity and firmness in pulp during three different stages were also assayed. The Northern analysis showed that the MA-Gal transcripts in pulp were detected at low level at preclimacteric stage, while an increasing progression was observed during fruit ripening, and the highest transcript amount was found at the postclimacteric stage. These results suggest that MAGal may play a role during banana fruit ripening and softening.
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