Nucleotide and deduced amino acid sequences of Rhizobium meliloti 102F34 lacZ gene: comparison with prokaryotic β-galactosidases and human β-glucuronidase

The nucleotide (nt) sequence of a 2.57-kb Sau3A. fragment carrying the Rhizobium meliloti β-galactosidase (βGal)-encoding gene (RmlacZ) was determined. An open reading frame (ORF) of 2.26 kb was identified which encoded a 755-amino-acid (aa) polypeptide with a calculated molecular mass of 84 141 Da, in fair agreement with the value of 88 kDa determined by SDS-PAGE. The deduced N-terminal aa sequence was confirmed by direct sequencing of electro-phoretically purified R. meliloti βGal. The size of the native R. meliloti βGal was approx. 174 kDa. Similarities were found between the aa sequence of the R. meliloti βGal and those from Clostridium thermosulfurogenes EM1 and Agrobacterium radiobacter, as well as human β-glucuronidase (βGlu). Comparisons with βGal from Escherichia coli, Klebsiella pneumoniae, Lactobacillus bulgaricus and Kluyveromyces lactis found only weak similarities; however, the putative active site residues appear to be conserved. The RmlacZ sequence is flanked by two partially sequenced ORFs, which show aa sequence and organisational similarities to the previously reported lac operon in A. radiobacter.