NUCLEOTIDE AND DEDUCED AMINO-ACID-SEQUENCES OF RHIZOBIUM-MELILOTI 102F34 LACZ GENE - COMPARISON WITH PROKARYOTIC BETA-GALACTOSIDASES AND HUMAN BETA-GLUCURONIDASE

The nucleotide (nt) sequence of a 2.57-kb Sau3A fragment carrying the Rhizobium meliloti beta-galactosidase (beta Gal)-encoding gene (RmlacZ ) was determined. An open reading frame (ORF) of 2.26 kb was identifie d which encoded a 755-amino-acid (aa) polypeptide with a calculated mo lecular mass of 84141 Da, in fair agreement with the value of 88 kDa d etermined by SDS-PAGE. The deduced N-terminal aa sequence was confirme d by direct sequencing of electrophoretically purified R. meliloti bet a Gal. The size of the native R. meliloti beta Gal was approx. 174 kDa . Similarities were found between the aa sequence of the R. meliloti b eta Gal and those from Clostridium thermosulfurogenes EM1 and Agrobact erium radiobacter, as well as human beta-glucuronidase (beta Glu). Com parisons with beta Gal from Escherichia coli, Klebsiella pneumoniae, L actobacillus bulgaricus and Kluyveromyces lactis found only weak simil arities; however, the putative active site residues appear to be conse rved. The RmlacZ sequence is flanked by two partially sequenced ORFs, which show aa sequence and organisational similarities to the previous ly reported lac operon in A. radiobacter.