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Characterization of Strong Polar Mutations in a Region Immediately Adjacent to the L-Arabinose Operator in Escherichia coli B/r¹

^a Department of Biological Sciences, University of Delaware, Newark, Delaware 19711

ABSTRACT

Seven L-arabinose-negative mutations are described that map in three genetically distinct regions immediately adjacent to the araO (operator) region of the L-arabinose operon. All seven mutants revert spontaneously, exhibit a cis-dominant, trans-recessive polarity effect upon the expression of L-arabinose isomerase (gene araA), and fail to respond to amber, ochre, or UGA suppressors. Three of these mutants exhibit absolute polarity and are not reverted by the mutangens 2-aminopurine, diethyl sulfate, and ICR-191. These may have arisen as a consequence of an insertion mutation in gene araB or in the initiator region of the L-arabinose operon. The four remaining mutants exhibit strong but not absolute polarity on gene araA and respond to the mutagens diethyl sulfate and ICR-191. Three of these mutants are suppressible by two independently isolated suppressors that fail to suppress known nonsense codons. Partially polar Ara⁺ revertants with lesions linked to ara are obtained from three of the same four mutants. These polar mutants, their external suppressors, and their partially polar revertants are discussed in terms of the mechanism of initiation of expression of the L-arabinose operon.

¹ One portion of this work was submitted by M. E. to the Department of Biological Sciences, Univ. of Delaware, in partial fulfillment of the requirements for the Doctor of Philosophy degree. A second portion of this work was submitted by B. G. to the same department in partial fulfillment of the requirements for the Bachelor of Arts degree, with distinction.

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