Crystallization and preliminary crystallographic analysis of RepA1, a replication control protein of the RepFIC replicon of enterotoxin plasmid EntP307

Historical Events That Spawned the Field of Plasmid Biology

This chapter revisits the historical development and outcome of studies focused on the transmissible, extrachromosomal genetic elements called plasmids. Early work on plasmids involved structural and genetic mapping of these molecules, followed by the development of an understanding of how plasmids replicate and segregate during cell division. The intriguing property of plasmid transmission between bacteria and between bacteria and higher cells has received considerable attention. The utilitarian aspects of plasmids are described, including examples of various plasmid vector systems. This chapter also discusses the functional attributes of plasmids needed for their persistence and survival in nature and in man-made environments. The term plasmid biology was first conceived at the Fallen Leaf Lake Conference on Promiscuous Plasmids, 1990, Lake Tahoe, California. The International Society for Plasmid Biology was established in 2004 (www.ISPB.org).

Interactions of the RepA1 protein with its replicon targets: two opposing roles in control of plasmid replication

By studying the interaction of derivatives of RepFIC miniplasmids, we were able to demonstrate that under certain conditions the RepA1 initiator protein inhibits plasmid replication. An analysis of cloned derivatives whose replication is inhibited by the RepA1 protein revealed the existence of two areas of the RepFIC genome that interact with RepA1 in the inhibition reaction. One of these areas, which occurs in the origin region, was explored by in vivo methylation protection footprinting studies. The protected area was 200 bp long and showed a definite periodicity of protected and hypersensitive sites, suggesting that RepA1 promotes a topological change in the RepFIC genome. The significance of our results is discussed in the context of plasmid replication control.

Role of the RepA1 protein in RepFIC plasmid replication

Using a sensitive primer extension technique, we have carried out studies to localize the start site of replication of the replicon RepFIC. In the course of these studies, we have found evidence that supports the hypothesis that transcription is an integral component of the initiation of replication. On the basis of our findings, we suggest that the transcript is processed to act as a primer, and therefore we propose that the transcript has a dual role as primer of replication and mRNA for the RepA1 protein. We present a model, based on our evidence, for the initiation of replication of the replicon RepFIC. This model provides as well an alternative explanation for what has been called the cis action of RepA1, and we show that RepA1 may act in trans as well as in cis.