CHARACTERIZATION OF CIS-ACTING MUTATIONS IN THE FIRST ATTENUATOR REGION OF THE BACILLUS-SUBTILIS PYR OPERON THAT ARE DEFECTIVE IN PYRIMIDINE-MEDIATED REGULATION OF EXPRESSION

A transcriptional attenuation mechanism for the regulation of pyr oper on expression in Bacillus subtilis in which the PyrR regulatory protei n binds pyr mRNA at three sites with similar sequences to cause transc ription termination in response to elevated pyrimidine nucleotide pool s has been proposed (R. J. Turner, Y. Lu, and R. L. Switzer, J. Bacter iol, 176:3708-3722, 1994), Twenty-seven mutants with cis-acting defect s in the repression by pyrimidines of beta-galactosidase expression of a pyr-lacZ fusion-integrant were isolated as blue colonies on X-Gal b romo-4-chloro-3-indolyl-beta-D-galactopyranoside) agar plates containi ng uracil and uridine after UV irradiation or treatment with mutagens or following mutD mutagenesis. These mutants showed normal repression of the chromosomal pyr operon by exogenous pyrimidines. Sequence analy sis revealed 12 unique sites of mutation, which occurred in the conser ved putative PyrR binding sequence (10 of the 12) or in the stem of th e transcriptional terminator structure. These mutants strongly support the proposed model for regulation of the pyr operon.