EXPRESSION AND IDENTIFICATION OF THE STRA-STRB GENE PAIR FROM STREPTOMYCIN-RESISTANT ERWINIA-AMYLOVORA

Deletions in either of the genes in the sti A-strB gene pair of Erwinia amylovora plasmid pEa34 resulted in a dramatic decrease in streptomycin resistance (Sm-R), but Sm-R was restored to high levels by complementation. When strA and strB were cloned separately on a lacI(q)/Ptac-based expression vector in Escherichia coli, only the protein encoded by strA was produced. When a strong Shine-Dalgarno sequence was introduced and the start codon was located outside the double-stranded region of the stable stem-loop, the protein encoded by strB was produced. Biochemical analysis of the respective phosphorylated products demonstrated thats strA and strB encoded aminoglycoside-3 -phospho transferase (APH(3'')-Ib) and aminoglycoside-6-phosphotransferase (APH(6)-Id), respectively, These data suggest that the high-level Sm-R in bacteria containing strA and strB is due to the combined action of the two enzymes.