Genetic instability in Streptomyces ambofaciens DSM 40697 is correlated with genomic instability characterized by multiple rearrangements (deletions and/or amplifications) occurring in a large unstable region. We have focused on one of the two amplifiable DNA loci which were mapped in this region: the amplifiable unit of DNA locus 6 (A UDd). The nucleotide sequence of one A UD6 fragment of 1-9 kb reveals the presence of two open reading frames (OW1 and ORF2) on the basis of the typical Streptomyces base composition at each of the three positions within codons. ORFl shows some similarity with a gene encoding a regulatory protein. The presence of potential genes in this unstable locus was unexpected because deletions occurred with high frequency within this region in the genetic instability-derived mutant strains. However, transcription analyses by S1 nuclease protection experiments on the wild-type strain showed transcription of both ORFl and ORF2. Moreover, the amplified strain reveals increased transcription of ORFl but no transcription of ORF2. The amplification therefore results in a switch in transcription. The unstable region of S. ambofaciens DSM 40697 therefore is not a 'silent' region because at least some loci are transcribed.