The sequence of the respective DNA regions downstream of the 20S proteasome structural genes prcB1A1 (6 kb) and prcB2A2 (3.3 kb) of Rhodococcus erythropolis NI86/21 were determined. A highly conserved gene organization was observed between the two clusters which differed significantly in G + C content (68.8% versus 62.6%). Several ORFs were homologues of putative genes previously identified by genomic sequencing of the equivalent DNA in the related nocardioform actinomycete, Mycobacterium leprae, and thought to be specific for this pathogen. Three ORFs (ORF8(1), ORF8(2), ORF12[1]) without a counterpart in M. leprae were found. No significant homology to known sequences including proteasome-related gene products was detected, except for ORF9(1) and ORF9(2) which display a high level of sequence identity with a partially sequenced ORF in Streptomyces chrysomallus. These downstream ORFs also show a significant level of sequence homology with the ORF6(1) and ORF6(2) which are located upstream of the proteasome structural genes in the respective clusters.