| One hallmark of development in both eukaryotic and prokaryotic organisms is that morphological checkpoints are necessary to ensure proper cell differentiation. An ideal system to study these events is Bacillus subtilis, an experimentally tractable organism that undergos the simple developmental process of sporulation, which results in the formation of two cells with distinct identities (the forespore and the mother cell) from one parental cell. During sporulation, two distinct morphological events serve as checkpoints for cell specific gene expression: polar septation allows activation of the forespore-specific transcription factor sigmaF , while the completion of the phagocytosis like process of engulfment allows activation of the second forespore-specific transcription factor sigma G. My thesis research demonstrates that SpoIIQ serves as a morphological checkpoint for both events.; At the onset of my studies, SpoIIQ was identified as a protein essential for migration of the mother cell membrane across the cell pole during engulfment. My studies demonstrated that SpoIIQ is only conditionally required for engulfment, but is essential for expression of certain genes under the control of the first forespore-specific sigma factor, sigmaF. At the completion of these studies, it was unclear if SpoIIQ was directly involved in engulfment, or if the conditional engulfment defect of the spoIIQ null was due to alterations in forespore-specific gene expression. Further genetic analysis of SpoIIQ demonstrated that SpoIIQ was separately required for engulfment (under certain conditions), expression of some sigmaF-dependent genes (including that which encodes the second forespore transcription factor sigma G), as well as activation of the second forespore-specific sigma factor, sigma G, upon the completion of engulfment. These results suggest that SpoIIQ serves as a morphological checkpoint coupling the ability of a sporangium to engulf to its ability to express sigmaG, and the completion of engulfment to activation of sigmaG.; My work also showed that MinC and MinD, two proteins essential for division site selection, are also essential for forespore-specific gene expression. Lastly, biochemical analysis of SpoIIQ, and two other engulfment proteins, SpoIID, SpoIIP, demonstrated that SpoIID and SpoIIP are cell wall hydrolases, giving insight into a potential mechanism behind membrane movement during engulfment. |
