Nonsense mutants in the regulator gene araC of the L-arabinose system of Escherichia coli B-r.

HE L-arabinose-gene complex in Escherichia coli B/r is a regulon (MAAS and CLARK 1964) corisisting of at least two operons controlled by regulator gene araC. One operon consists of structural genes, araD (L-ribulose 5-phosphate 4-epimerase), araA (L-arabinose isomerase), and araB (L-ribulokinase) , and controlling sites, araZ (initiator) and araO (operator). These genetic units are linked in that order between thr and gene araC (Figure 1 ) . The other operon concerned with L-arabinose transport may consist of gene araE with presumably similar controlling sites. Gene araE is not linked to the thr-ara-leu region of the coli DNA but is linked to thy. (ENGLESBERG, unpublished data). Gene araC exerts a positive control over the L-arabinose structural genes B, A, and D. The essential aspects of this positive control system are that the expression of the structural genes is normally “turned off’ and that the presence of a specific cytoplasmic regulator gene product is necessary for structural gene expression (ENGLESBERG et al. 1965; SHEPPARD and ENGLESBERG 1966, 1967). Evidence recently published ( ENGLESBERG and SQUIRES 1968; ENGLESBERG, SHEPPARD, SQUIRES and MERONK 1969a; ENGLESBERG, SQUIRES and MERONK 1969b; KESSLER and ENGLESBERG 1969), strongly supports the model for positive control as diagrammed in Figure 2. In this paper we describe an analysis of eleven araCmutants (L-arabinose nonutilizing, pleiotropic negative, recessive) and their revertants. All the C-