A transcriptomic analysis of the mycobacteriophage D29 genome reveals the presence of novel stoperator-associated promoters in its right arm.

摘要

Mycobacteriophage D29 is a lytic phage that infects various species of Mycobacterium including M. tuberculosis. Its genome has 77 genes distributed almost evenly between two converging operons designated as left and right. Transcription of the phage genome is negatively regulated by multiple copies of an operator-like element known as stoperator that acts by binding the phage repressor Gp71. The function of the D29 genes and their expression status are poorly understood and therefore we undertook a transcriptome analysis approach to address these issues. The results indicate that the average transcript intensity of the right arm genes was higher than of those on the left, at the early stage of infection. Moreover, the fold increase from early to the late stage was found to be less for the right arm genes than for the left. Both observations support the prediction that the right arm genes are expressed early whereas the left arm ones are expressed late. The analysis further revealed a break in the continuity of the right arm operon between 89, the first gene in it, and 88, the next. Gene 88 was found to be expressed from a newly identified promoter located between 88 and 89. Another new promoter was found upstream of 89. Thus, the promoter Pleft, identified earlier, is not the only one that drives expression of the right arm genes. All these promoters overlap with stoperators, with which they share a conserved sequence motif, TTGACA, commonly known as the -35 promoter element. We demonstrate mutually exclusive binding of RNA polymerase and Gp71 to the stoperator-promoters and conclude that stoperators can function as -35 promoter elements and that they can control gene expression not only negatively as was believed earlier but in many cases positively as well.