The genes encoding the six pyrimidine biosynthesis enzymes from the thermophile caldolyticus were characterized by cloning and complementation in , and by nucleotide sequence analysis. Nine cistrons are clustered within an 11 kb region of the chromosome, the gene order being: orf1-pyrB-pyrC-pyrAa-pyrAb-orf2-pyrD-pyrF-pyrE. This organization of the cluster is very similar to that of the pyr operon of Different parts of the cluster were cloned in two orientations in the expression shuttle vector pHPS9. Complementation studies in established that expression of the genes was dependent on the vector-borne promoter, suggesting that they are part of an operon, and that the native promoter of the operon had not been cloned. The deduced amino acid sequence of the individual cistrons showed 49 to 78% identity with the corresponding cistrons. Measurements of the aspartate transcarbamylase (), orotidine monophosphate decarboxylase () and orotate phosphoribosyltransferase () levels in cells grown under different conditions indicated that expression of the operon is repressed 7–9-fold by addition of uracil to the growth medium. Based on the nucleotide sequence in the intercistronic region between and a regulatory mechanism involving transcriptional termination and antitermination is proposed to control expression of the operon.


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