Date of Award

7-31-2006

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

First Advisor

Gladys Alexandre - Chair

Second Advisor

John Houghton

Third Advisor

Robert Simmons

Fourth Advisor

Igor Zhulin

Fifth Advisor

P.C. Tai

Abstract

The ability to swim and navigate the surrounding environment confers an advantage to motile bacteria, allowing the occupation of niches that are optimum for survival and growth. Bacteria are too small to sense their environment spatially, so they must sense the environment temporally by comparing the past and present environments and altering their motility accordingly. Chemotaxis systems coordinate flagellar motility responses with temporal sensing of the environment. Chemotaxis is proposed to be involved in plant root colonization by directing soil bacteria toward root exudates of various cereals, promoting growth. The nitrogen-fixing alpha-proteobacterium Azospirillum brasilense utilizes chemotaxis to navigate its environment by integrating various environmental signals into a chemotaxis signal transduction pathway. In chemotaxis, transducers receive environmental sensory information and transmit the signal to the histidine kinase CheA, which relays the signal to the response regulator CheY. A novel chemotaxis transducer, Tlp1, has been identified and characterized as an energy sensor by constructing a tlp1 mutant and performing behavioral and root colonization assays. In order to adapt to changing environmental conditions, chemotactic microorganisms must employ a molecular “memory” comparing present environmental conditions to ones previously experienced and resetting the chemotaxis transducer to a prestimulatory status. A recently identified chemotaxis operon revealed a methyltransferase CheR and methylesterase CheB, comprising an adaptation system, suggesting that A. brasilense undergoes methylation-dependent taxis responses, contrary to previous reports. Chemotaxis and methanol release assays suggest that adaptation by methylation in locomotor behavior involves the presence of other unknown methylation systems, and the contribution of CheR and CheB to chemotactic and aerotactic responses is complex. There is growing evidence that chemotaxis-like signal transduction pathways control a myriad of other cellular processes regulated in a temporal fashion. This would convey an advantage to cells by allowing modulation of cellular processes based on slight changes in environmental conditions and provide checkpoints for energetically consuming processes. Mutations in components of the chemotaxis-like signal transduction system revealed differences in cell size and exopolysaccharide production. This work shows that the signal transduction pathway of A. brasilense modulates cell length in response to changes in nutrient conditions, independently of growth rate.

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Biology Commons

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