Summary: From a partial 3AI library of A3(2) DNA in pIJ916, two hybrid plasmids pGX1 and pGX2 were isolated that complemented A3(2) or arginine auxotrophs. Subcloning DNA from pGX1 in the expression vector pRK9 containing the promoter gave rise to one plasmid, pZC2, that complemented auxotrophs, and another, pZC1, that complemented only the first three. The plasmids were markedly unstable in the various complemented hosts, to varying extents; pZC1 was characterized further as providing the stablest host/plasmid combinations. deletion of part of the vector's promoter did not affect complementation of the and auxotrophs, implying that the A3(2) genes may be expressed from their own promoter. The promoter-less plasmids included isolates, such as pZC177, that had suffered extensive further deletion without loss of complementing ability. Extracts of an auxotroph carrying pZC177 showed ornithine acetyltransferase activity, indicating that the complementing gene is of the type. The complementation properties of deletion derivatives of pZC177 indicated the gene order Part of and the upstream region were sequenced; an ORF was identified whose predicted product showed appreciable homology with the and ArgC polypeptide. Upstream of this ORF a consensus-type promoter and ribosome binding site could be discerned; overlapping its promoter was a sequence with homology to arginine operators in these two other organisms. An frameshift in had a polar effect on expression in of and , suggesting that the three genes are transcribed in the same direction, possibly as an operon.


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