Co-occurrence of amtR and AmtR-regulated genes in selected Actinobacteria . Genomic map of the highly conserved AmtR operon in K. radiotolerans (A), S. avermitilis (B) and M. smegmatis (C). The arrows indicate length and transcriptional orientation of annotated genes which are depicted by the respective orf number. Numbers in square brackets show the lengths of intergenic regions in bp. While the putative K. radiotolerans operon shows a subsequent series of genes with overlapping reading frames, the amtR gene is transcribed in opposite direction in S. avermitilis and the amino acid permease-encoding gene is located elsewhere, while in M. smegmatis, the amtR gene is not co-localized with the putatively AmtR-regulated genes (see Table 3 for details).