Expression of tylM genes during tylosin production: phantom promoters and enigmatic translational coupling motifs

In the genome of Streptomyces fradiae, the three tylM genes are codirectional with the upstream gene, tylGV. Although the introduction of transcriptional blocks into the tylM genes revealed that they are normally cotranscribed, expression of tylMI still persisted (albeit at a very low level) when either of the upstream genes, tylMII or tylMIII, was disrupted. Such expression apparently resulted from transcriptional initiation at spurious sites that probably contribute insignificantly, if at all, to promote activity in the wild type. Prior to the onset of tylosin production, tylMIII is transcribed independently of tylGV from an authentic promoter buried within tylGV. This latter observation is interesting given that the TGA stop codon of tylGV overlaps the GTG start codon of tylMIII. Evidently, terminally overlapping genes are not always translationally coupled.