Type I restriction systems: Sophisticated molecular machines (a legacy of Bertani and Weigle)

Restriction ezymes are well known as reagents widely used by molecular biol ogists for genetic manipulation and analysis, but these reagents represent only one class (type II) of a wider range of enzymes that recognize specifi c nucleotide sequences in DNA molecules and detect the provenance of the DN A on the basis of specific modifications to their target sequence. Type I r estriction and modification (R-M) systems are complex; a single multifuncti onal enzyme can respond to the modification stare of its target sequence wi th the alternative activities of modification or restriction. In the absenc e of DNA modification, a type I R-M enzyme behaves like a molecular motor, translocating vast stretches of DNA towards itself before eventually breaki ng the DNA molecule. These sophisticated enzymes are the focus of this revi ew, which will emphasize those aspects that give insights into more general problems of molecular and microbial biology. Current molecular experiments explore target recognition, intramolecular communication and enzyme activi ties, including DNA translocation. Type I R-M systems are notable for their ability to evolve new specificities, even in laboratory cultures. This obs ervation raises the important question of how bacteria protect their chromo somes from destruction by newly acquired restriction specifities. Recent ex periments demonstrate proteolytic mechanisms by which cells avoid DNA break age by a type I R-M system whenever their chromosomal DNA acquires unmodifi ed target sequences. Finally, the review will reflect the present impact of genomic sequences on a field that has previously derived information almos t exclusively from the analysis of bacteria commonly studied in the laborat ory.