Cloning and expression analysis of beta-galactosidase gene related to softening of banana (Musa sp.) fruit.

Beta-galactosidase is thought to be involved in fruit softening through cleaving beta(1-->4) galactan bonds in cell wall hemicellulose. To study the relationship between fruit softening and beta-Gal during banana (Musa sp.) fruit ripening, a beta-Gal cDNA fragment, named MA-Gal, has been cloned from banana fruit pulp using RT-PCR 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 beta-galactosidase expressed in fruit ripening. The MA-Gal putative amino acids have five homologous domains (typical eukaryotic beta-Gals have seven domains), contain the GGPIILSQIENEY(F) motif, which is a presumptive catalytic active site conserved in all beta-Gals. Phylogenetic analysis showed that MA-Gal was in plant beta-Gal group I, which was expressed in fruit tissue. The changes in beta-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 MA-Gal may play a role during banana fruit ripening and softening.