Doctor of Philosophy (PhD)


Biological Sciences

Document Type



Conjugation in Streptomyces invokes a unique mechanism involving few plasmid-encoded loci. Transfer of circular plasmid pIJ101 requires only a tra gene, and a cis-acting locus clt. Here, I investigated whether the transfer genes of pIJ101 could promote transfer of a linear plasmid. While pIJ101 Tra could transfer circularized versions of the linear plasmid containing clt, the linear plasmid itself could not be transferred efficiently by Tra. All plasmids isolated from transconjugants appeared to be circular regardless of the configuration of the plasmid in the initial donor cells. However the linear plasmid could be transferred in linear form from JW1, which contains the conjugative linear plasmid SLP2. Evidence that linear plasmids transferred from strains containing only SLP2 strongly suggest that TraSLP2 mediated the transfer of heterologous linear plasmid and pIJ101, and Tra is not responsible for transfer of the linear DNA molecules from JW1.These results indicate that Tra of pIJ101 shows specificity for the circular configuration and imply that efficient transfer of linear plasmids may require additional functions compared with circular plasmids. Certain strains of the bacterial sweet potato pathogen Streptomyces ipomoeae produce the bacteriocin ipomicin, which inhibits other sensitive strains of the same species. Here, I show that group III inhibitor ipomicin stably accumulates in culture supernatants of S. ipomoeae in a growth-regulated manner that does not coincide with the pattern of expression of the ipomicin structural gene ipoA. Similar growth-regulated production of ipomicin in Streptomyces coelicolor containing the cloned ipoA gene was found to be directly dependent on translation of the TTA codon in ipoA by the bldA leucyl tRNA. These results suggest that bldA-dependent translation of the S. ipomoeae ipoA gene leads to growth-regulated production of the ipomicin precursor, which upon processing to the mature form and secretion, stably accumulates in the extracellular environment. An apparently inactive form of ipomicin protein is produced in most members of susceptible strains of S. ipomoeae, suggesting that further post-translational modification of ipomicin protein in the group III strains results in bacteriolytic activity.



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Committee Chair

Pettis, Gregg S.