Spontaneous mutations in the galactose operons of Streptomyces coelicolor A3 (2) and Streptomyces lividans 66

Mutations can be divided into two classes: point mutations (base changes, frame shifts) and DNA rearrangements (deletions, insertions, inversions etc.). In Escherichia coli K 12, DNA insertions account for up to 40% of spontaneous mutations that inactivate genes (for review, see CULLUM 1985) and the Insertion Sequences involved can also mediate deletions and inversions. We started to study spontaneous mutations in Streptomyces in the expectation of isolating transposable elements and chose the galactose genes as convenient system. When selection is made for resistance to the galactose analogue 2-deoxygalactose (2dgalR), gal K mutants are obtained that are defective in the enzyme galactokinase (Kendall et al. 1987). We isolated spontaneous 2dgalR mutations in the closely related strain S. coelicolor A 3 (2) and S. lividans 66 and cloned the gal K gene of the former strain. Adams et al. (1988) have cloned and sequenced the gal genes of S. lividans 66 and shown that they form an operon, the restriction map appears to be identical to that of S. coelicolor A 3 (2). In this paper we describe the characterisation of spontaneous 2 dgalR mutants in S. coelicolor A 3 (2) and S. lividans 66.