Figure 43-1: Phage λ: its genome and transcription circuits. Upper. The phage DNA enters the bacterium as a linear molecule but circularizes via its cohesive ends. Genes that encode related functions are clustered in the lambda genome. Genes within the "central region", identified by the solid segment, are inessential for propagation of the phage. att, located within the central region identifies the site at which site-specific recombination integrates the phage genome into the bacterial chromosome. The map is not drawn to scale. Lower. Transcription of the genome proceeds initially from two promoters, pL and pR. At the earliest stage of infection, transcripts initiated at pL and pR (I) terminate at specific signals, υ, (some rightward transcripts initiated at pR reach the second termination signal). Only in the presence of the N gene product does most transcription proceed beyond the termination signals into the downstream genes. The consequent synthesis of Q protein allows transcription of the late genes (II) to proceed from p through the adjacent termination signal into the lysis genes and the genes that specify the components of the phage head and tail. Transcription of the repressor gene (cI) during the establishment of lysogency is initiated at a promoter pRE situated to the right of cro. Repressor synthesis is maintained by autogenously controlled transcription from pRM, a promoter that overlaps OR. l major promoters, υ major termination signals susceptible to N- and/or Q-dependent antitermination; n major termination signals that block transcription of N- and/or Q-modified RNA polymerase. (for better image quality, click here for tif image, which may be quite large; may also require screen refreshing plus special software to view; or view larger tif in separate window)

Figure 43-1, Figure 43-2, Figure 43-3, Figure 43-4

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