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POWER, AND NANCY LEE. Positive control of enzyme synthesis by gene C in the L-arabinose system. J. Bacteriol 90:946-957. 1965.-The L-arabinose gene complex consists of genes D, A, B, and C, linked in that order between the markers thr and leu, and an unlinked gene E. Genes D, A, B, and E are the structtural genes for three inducible enzymes and permease, respectively. Gene C, with two mutant alleles, C-and Cc, is the regulatory gene exhibiting positive and negative control. Cmutants are deficient and Cc mutants are constitutive for all three enzymes and permease. Complementation analysis, employing sexual merozygotes (A-C+ X A+C-), with six different Cmutants, demonstrates that C-is recessive to C+ (positive control). A total of 61 Cc mutants, isolated as clones resistant to D-fucose inhibition, are linked to the leu ara region of the chromosome, and the 22 Cc mutants that were analyzed in detail mapped within the C gene among the Cmutanit sites. cO mutants produce various but coordinate levels of the two enzymes measured, and permease. Complementation analysis (A-Cc X A+C-, A-CC X A+C+) shows that Cc is dominaint to C-(positive control) and recessive to C+ (negative control). Deletion mutanits that extend into the C gene are L-arabinose permease-negative, thus stupporting the positive regulatory role of the C gene. The name "activator gene" is proposed for genes of the C type to accentuate their positive role in gene expression. A working model consistent with these results is presented.

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Abstract.-The product of the regulator gene araC in the L-arabinose gene complex exists in two functional states: P1, the repressor, and P2, the activator, presumably in equilibrium with each other, and with P1 and P2 attached to their respective controlling sites, araO, the operator, and araI, the initiator. The controlling sites are linked in the following order with respect to genes araB and araC: BIOC. Two C gene deletions (A719 and A766) serve to define the newly described araO site, and to place it adjacent to the left end of the C gene. We have suggested that deletion 719 deletes, in addition to the C gene, part or all of the araO site. Deletion 766 leaves the araO site intact. Complementation analysis employing stable merodiploids indicates that the repressoroperator site function is epistatic over the activator-initiator site function. It is necessary both for activator (P2) to be present and for repressor (P1) to be absent at their respective controlling sites (araI and araO) for full expression of the L-arabinose operon.The L-arabinose gene-enzyme complex in Escherichia coli B/r has been shown to consist of structural genes araD, araA, araB, controlling site araI (the initiator), and regulatory gene araC, linked in that order between the markers thr and leu and the unlinked gene, araE, concerned with the active transport of L-arabinose, with its corresponding controlling sites.1-3 Several lines of evidence have been presented indicating that gene araC controls in a positive fashion the expression of the structural genes in this system. A detailed analysis of this evidence is presented in another publication.4 Briefly, it has been shown that deletions1-3 and nonsense mutations5 of the C gene lead to the productioM of a pleiotropic-negative phenotype (C-) which is recessive (cis and trans) to both the wild-type (C+) and constitutive (Cc) alleles of that gene. The findings that unlinked suppressors restore activity of C-mutants and that some revertants of these C-mutants form a thermolabile C gene product indicate that the activator is at least partially a protein molecule.5 No support has been found for the existence of another L-arabinose regulatory gene producing a repressor specific for the L-arabinose system.4 5 Several lines of evidence indicate that the controlling sites are located in the region between genes B and C: (1) polarity is in the direction BAD;6 7 (2) gene araC is not part of the BAD operon, since nonsense and deletion mutations in the araC gene'-3 5 and deletion mutations that end within the araC gene and the leu operon do not affect the L-arabinose-gene araC control of the L-arabinose operon, while deletion mutations that end within the araB gene and the leu operon remove the remaining structural genes of the operon from the control of L-arabinose and gene araC 1100