UNUSUAL G-CAT SUPRA-OPERONIC CLUSTER IN THE ACINETOBACTER-CALCOACETICUS CHROMOSOME(C CONTENT AND CODON USAGE IN CATIJF, A SEGMENT OF THE BEN)

The nucleotide (nt) sequence of a 5.3-kb DNA segment containing the Gr am(-) Acinetobacter calcoaceticus catBCIJFD operon is reported. This i nformation completes determination of a 16-kb nt sequence containing t he twelve ben and cat structural genes encoding enzymes required for c atabolism of benzoate via the beta-ketoadipate pathway. Many of these genes can be traced to a common ancestry with genes from other organis ms containing DNA with widely divergent G+C content. The A. calcoaceti cus ben and cat genes are arranged in a supra-operonic cluster contain ing one known regulatory gene and three additional open reading frames (ORFs) that may have regulatory functions. Thirteen of the ben and ca t genes, including the three ORFs with unknown function, are typical f or A. calcoaceticus in that they possess a G+C content of 44.9 +/- 2.5 %. Three exceptional A. calcoaceticus genes (catI, catJ and catF) poss ess G+C contents of 56.5 +/- 1.3%. These differences in G+C content ar e reflected in the distinctive patterns of codon usage shared by catI, catJ and catF. Thus, the catIJF region, known to exchange genetic inf ormation with the pcaIJF region in the same chromosome directing isofu nctional proteins associated with the beta-ketoadipate pathway, has av oided the evolutionary forces that conferred characteristic G+C conten t upon the other ben and cat genes in A. calcoaceticus.