DNA replication initiates at defined replication origins along eukaryotic chromosomes, ensuring complete genome duplication within a single S-phase. A key feature of replication origins is their ability to control the onset of DNA synthesis mediated by DNA polymerase-α and its intrinsic RNA primase activity. Here, we describe a novel origin-independent replication process that is mediated by transcription. RNA polymerase I transcription constraints lead to persistent RNA:DNA hybrids (R-loops) that prime replication in the ribosomal DNA locus. Our results suggest that eukaryotic genomes have developed tools to prevent R-loop-mediated replication events that potentially contribute to copy number variation, particularly relevant to carcinogenesis.D uring transcription, DNA acts as a template for the synthesis of the nascent RNA. RNA synthesis is accompanied by the generation of positive and negative DNA supercoiling in front of and behind the transcription machinery, respectively (1). Unwinding of the DNA double helix by negative supercoiling may allow the RNA to hybridize to its DNA template behind the elongating RNA polymerase, leading to R-loops (2). Other elements that could potentiate R-loop accumulation include RNA: DNA hybrid-facilitating DNA sequences, such as G-quadruplex structures (3) or nicks in the nontemplate DNA strand (4).Eukaryotic cells need to control R-loop formation to avoid replication impairment, genome instability, and life span shortening mediated by such intermediates (5-10; reviewed in ref. 11). To do so, cells catalyze the relaxation of supercoiled DNA by type I topoisomerases (12-15), thus preventing replication fork reversal (16), DNA overwinding with the potential to block replication fork progression (17), DNA unwinding (18), or R-loop-mediated blocks of ribosomal RNA synthesis (19). Other enzymatic activities involved in R-loop processing include ribonuclease H (RNase H) activities, DNA-RNA helicases, such as Sen1/senataxin (20, 21), or Ataxin-2 RNA-binding protein Pbp1 (10). The ribonuclease activity of Saccharomyces cerevisiae RNases H1 and H2 specifically cleaves the RNA moiety of the RNA:DNA hybrid structure (22), whereas RNase H2 and topoisomerase 1 (Top1) can also process ribonucleotides in duplex DNA (23,24).Notably, R-loops are required to initiate mitochondrial DNA replication (25) and pioneering studies connected R-loops to origin-independent replication in prokaryotic systems (26,27). For example, DnaA-dependent initiation of DNA replication at the Escherichia coli oriC replication origin can be overcome in the absence of RNase H1 (28, 29). As a consequence, rnhA mutants can survive complete inactivation of oriC by transcriptiondependent activation of so-called oriK sites (30, 31), although candidate oriK sites have been identified only recently (32). Additional evidence for R-loop-primed replication was given by the observation that rnhA mutants are prone to an increase in mutation and DNA amplification events if origin activity is suppressed. These events required remov...

one should not perhaps expect much informa,tion on more recent work in phamacognosy and additions to the materia m e d i a such as alginates, Ammi Visnaga, the Holarrhena alkaloids, Eryfhrina alkaloids and rutin, but one would expect information on ,thyroid and pancreas, both of which %re in the new syllabus for the degree of Bachelor of Pharmacy. Apart from these omissions the author's aim is well fulfilled in this book.J. W. FAIRBAIRN.