Los fagos pueden clasificarse de acuerdo a su desarrollo en dos grupos: los fagos virulentos y los fagos temperados. Entre los genes P y Q se encuentra un segundo terminador tR2. Este promotor esta localizado entre el gen Q y el gen R. Se seleccionaron mutantes de E. Adicionalmente ambos factores interaccionan directamente con la ARN polimerasa. El mecanismo por la cual antitermina es diferente al de N.
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The development of molecular cloning techniques, ironically instigated largely by phage lambda researchers, allowed many phage workers to switch their efforts to other biological systems. Nonetheless, since that time the ongoing study of lambda and its relatives have continued to give important new insights. During those decades lambda was one of the few experimental "organisms" that was sufficiently experimentally accessible to be viewed as potentially completely understandable.
Its genome was small enough not to be overwhelming, yet it was capable of making a decision of whether to lysogenize or grow lytically and was clearly complex enough to be interesting and informative on many fronts. In particular it was unique in that a genetic and molecular understanding of the control of gene expression seemed to be within almost immediate reach.
Those of us lucky enough to be involved in the study of phage lambda during this time were tremendously excited about future possibilities and threw ourselves into the work with all the energy we had. Lambda was originally discovered in by Esther Lederberg at the University of Wisconsin Madison , when she serendipitously found it was released from the laboratory Escherichia coli strain K after ultraviolet irradiation. Lambda became a more important and approachable experimental system when Allan Campbell isolated a set of suppressible nonsense mutants of lambda or amber mutations, originally called suppressor sensitive or sus mutations that identified genes essential for its lytic growth.
Additional amber mutations isolated by Sandy Parkinson and Goldberg and Howe and prophage mutations isolated by Clarence Fuerst Mount et al. We note that two additional genes, Rz and Rz1 should probably also be considered essential genes even though plaques often form in their absence. They are absolutely required for disruption of the outer membrane and lysis unless external physical solution forces that are often present during laboratory growth help to destabilize the outer membrane of infected cells Berry et al.
Only the small cro gene was not found by the above random mutant hunts. It was discovered during the genetic analysis of the control of lysogeny Eisen et al.
Recombination frequency and deletion mapping of these and various promoter and operator mutations isolated by others during this time allowed the construction of a detailed genetic map.
Detailed study of the phenotypes of phage mutants defective in known, mapped genes led to an early overall picture of the lambda lifecycle. Indeed, the careers of both authors here started at about the same time with the analysis of lambda virion morphogenesis by utilizing the various amber mutants to identify the proteins expressed from the lambda morphogenetic genes and determining the nature of the defects in their absence - SRC in the laboratory of Dale Kaiser at Stanford U.
Happily, rather than generating animosity, our early competition in this arena led to mutual respect and a lifelong friendship. We believe that the clever use of forward genetic selections and screens to isolate informative mutations and combinations of mutations and the use of these molecular genetic experiments to deduce the underlying molecular mechanisms reached its zenith in the study of phage lambda during this period. Those were heady days with lambda running at the front of the scientific pack.
We discuss here early experiments that led to our current understanding of phage lambda and molecular biological processes in general. We also mention recent developments to show how far the field has come in the past 60 years we assume a basic knowledge of the phage lambda life cycle; see Hendrix et al.
We include discussion of phages related to lambda, which are commonly referred to as "lambdoid" phages. This ill-defined and often incorrectly used term refers phages with very similar lifestyles to lambda and genomes that are mosaically related to lambda. This definition included the notion that a lambdoid phage is capable of recombination with lambda itself to produce a functional hybrid phage, as was first shown with phage by Kaiser and Jacob Recent phage genome sequences have caused an expansion of this term to mean a phage with the same functional gene order as lambda and that carries patches of nucleotide sequence homology with lambda or another lambdoid phage.
Thus, in theory a single recombination event between lambdoid phages could give rise to a fully functional phage that has all the necessary genes Hendrix, ; Casjens, ; Hendrix and Casjens, ; Grose and Casjens, In this discussion we use lambdoid to include for example, the three phages HK97, P22 and N15, which typify three of the best-studied lambdoid groups that have significant differences from lambda.
Citations are in general not meant to bestow credit for the original discoveries but to allow the reader access to the literature. Other more detailed historical treatments of some of the topics covered below can be found in the books Bacteriophage lambda Hershey, and Lambda II Hendrix et al.
Historical importance and recent progress The lambda genome Phage lambda DNA was one of the earliest model systems for studying the physical nature of DNA and genes, and a substantial amount of early research examined the hydrodynamic properties of the lambda chromosome with the goal of, among other things, determining its absolute molecular weight. At the same time methods now considered to be primitive were devised for separating the two halves of the lambda chromosome and for mapping genes identified genetically to these halves and to smaller physical intervals reviewed by Davidson and Szyblaski, Soon thereafter electron microscopic analysis of absolute length and of lambda DNA in heteroduplex with various altered lambda DNAs gave rise to a major step forward, the construction of a detailed physical map of lambda DNA - a gene map in base pairs bps rather than recombination frequency units Fiandt et al.
Meanwhile, lambda contributed greatly to the ultimate mapping of DNA, the complete nucleotide sequence. It is not generally appreciated that the 12 bp lambda cohesive ends were the subject of the first direct nucleotide sequencing of a biological DNA. Ray Wu devised methodology for using DNA polymerase to fill in the cohesive ends with specifically labeled nucleotides, followed by analysis of the product to determine this exact 12 bp nucleotide sequence Wu and Kaiser, ; Onaga, This sequencing was not a trivial undertaking, as determining this 0.
Fourteen years later, Fred Sanger et al. This, along with anecdotal sequencing of various lambda genes and mutants of them by the laborious and more error-prone chemical sequencing method Maxam and Gilbert, in the previous few years, unleashed a rapid avalanche of studies relating the many extraordinary genetic studies on lambda from the previous decades to the genome sequence.
This in turn gave rise to an understanding of phage lambda genes and their expression that was unprecedented at the time reviewed in Hendrix et al. Figure 1 shows a map of the lambda chromosome; more detailed annotated maps of lambda, HK97, P22 and N15 can be found in the supplementary material of Hendrix and Casjens
Bacteriophage lambda: early pioneer and still relevant
Anatomy[ edit ] Bacteriophage lambda virion schematic. Protein names and their copy numbers in the virion particle are shown. The presence of the L and M proteins in the virion is still unclear. The whole particle consists of 12—14 different proteins with more than protein molecules total and one DNA molecule located in the phage head. However, it is still not entirely clear whether the L and M proteins are part of the virion.