A Family of at Least Seven β-Galactosidase Genes Is Expressed during Tomato Fruit Development

During our search for a cDNA encoding β-galactosidase II, a β-galactosidase/exogalactanase (EC 3.2.1.23) present during tomato (Lycopersicon esculentum Mill.) fruit ripening, a family of seven tomato β-galactosidase (TBG) cDNAs was identified. The shared amino acid sequence identity among the seven TBG clones ranged from 33% to 79%. All contained the putative active site-containing consensus sequence pattern G-G-P-[LIVM]-x-Q-x-E-N-E-[FY] belonging to glycosyl hydrolase family 35. Six of the seven single-copy genes were mapped using restriction fragment length polymorphisms of recombinant inbred lines. RNA gel-blot analysis was used to evaluate TBG mRNA levels throughout fruit development, in different fruit tissues, and in various plant tissues. RNA gel-blot analysis was also used to reveal TBG mRNA levels in fruit of the rin, nor, and Nr tomato mutants. The TBG4-encoded protein, known to correspond to β-galactosidase II, was expressed in yeast and exo-galactanase activity was confirmed via a quantified release of galactosyl residues from cell wall fractions containing β(1→4)-d-galactan purified from tomato fruit.