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

The nucleotide (nt) sequence of a 5.3-kb DNA segment containing the Gram(-) Acinetobacter calcoaceticus catBCIJFD operon is reported. This information completes determination of a 16-kb nt sequence containing the twelve ben and cat structural genes encoding enzymes required for catabolism of benzoate via the beta-ketoadipate pathway. Many of these genes can be traced to a common ancestry with genes from other organisms containing DNA with widely divergent G+C content. The A. calcoaceticus ben and cat genes are arranged in a supra-operonic cluster containing one known regulatory gene and three additional open reading frames (ORFs) that may have regulatory functions. Thirteen of the ben and cat genes, including the three ORFs with unknown function, are typical for A. calcoaceticus in that they possess a G+C content of 44.9 +/- 2.5%. Three exceptional A. calcoaceticus genes (catI, catJ and catF) possess G+C contents of 56.5 +/- 1.3%. These differences in G+C content are reflected in the distinctive patterns of codon usage shared by catI, catJ and catF. Thus, the catIJF region, known to exchange genetic information with the pcaIJF region in the same chromosome directing isofunctional proteins associated with the beta-ketoadipate pathway, has avoided the evolutionary forces that conferred characteristic G+C content upon the other ben and cat genes in A. calcoaceticus.