The Psu protein of bacteriophage P4 is an antitermination factor for rho-dependent transcription termination

A 0.7-kbp DNA fragment from bacteriophage P4 that contained the polarity suppression (psu) gene was cloned in an expression plasmid. Induction of the plasmid-borne psu gene resulted in the overproduction of a protein having the biological properties of the P4-induced polarity suppressor. In vivo, Psu protein acted in trans to suppress rho-dependent polarity in the late genes of an infecting P2 phage, in plasmid operons, and in the host chromosome. Psu action did not require the presence of other P2 or P4 phage genes. Psu caused efficient readthrough (antitermination) by Escherichia coli RNA polymerase at the rho-dependent terminators tR1 and TIS2, individually and in tandem, but did not affect termination at rho-independent sites. Neither the conserved antitermination sequence boxA nor any unique promoter or utilization sequence was required for Psu activity.     

Lack of a unique termination site for the first round of bacteriophage lambda DNA replication

From previous data on the first round of bacteriophage λcIIcIII DNA replication (Schnös & Inman, 1970) it is possible to estimate, by extrapolation, the position on circular λ DNA where bidirectional growing points meet. In the present study we have investigated whether this position occurs at a genetically defined site. To this end, replicative intermediates of λ mutants containing either deletions to the left of the replication origin, or one deletion plus a duplication to the right, were analyzed in the electron microscope. Our results indicate that: (i) leftward growing points can traverse the extrapolated termination point calculated from the λcIIcIII data, (ii) no discontinuity of either right or leftward growing fork position is observed, and (iii) the extrapolated termination points for these mutants are well removed from those calculated for λcIIcIII DNA. From these data we conclude that there is probably no unique termination site for the first round of λ DNA replication and that termination occurs simply by collision of the growing forks.     

Defining cosQ, the site required for termination of bacteriophage lambda DNA packaging

Bacteriophage lambda is a double-stranded DNA virus that processes concatemeric DNA into virion chromosomes by cutting at specific recognition sites termed cos. A cos is composed of three subsites: cosN, the nicking site; cosB, required for packaging initiation; and cosQ, required for termination of chromosome packaging. During packaging termination, nicking of the bottom strand of cosN depends on cosQ, suggesting that cosQ is needed to deliver terminase to the bottom strand of cosN to carry out nicking. In the present work, saturation mutagenesis showed that a 7-bp segment comprises cosQ. A proposal that cosQ function requires an optimal sequence match between cosQ and cosNR, the right cosN half-site, was tested by constructing double cosQ mutants; the behavior of the double mutants was inconsistent with the proposal. Substitutions in the 17-bp region between cosQ and cosN resulted in no major defects in chromosome packaging. Insertional mutagenesis indicated that proper spacing between cosQ and cosN is required. The lethality of integral helical insertions eliminated a model in which DNA looping enables cosQ to deliver a gpA protomer for nicking at cosN. The 7 bp of cosQ coincide exactly with the recognition sequence for the Escherichia coli restriction endonuclease, EcoO109I.     

Attachment site mutants of bacteriophage lambda

Insertion and excision of the chromosome of phage λ occurs by recombination at special regions of the phage and bacterial chromosomes known as attachment sites (alt's). We have isolated att mutants which display reduced recombination frequencies. The mutations are cis-dominant, trans-recessive, and can be crossed into a phage, bacterial or prophage att. These results suggest that the att's, although different over-all, include the same DNA sequence as part of their structure, and that the mutations reside in these sequences. Crosses between mutant and wild-type att's occasionally yield heterozygotes. This result suggests that recombination of the att's generates complementary single-strands via staggered nicks in these common sequences. Recombinant att's are then formed by the interannealing of single-strands of different att's followed by ligation.     

Control of bacteriophage lambda repressor establishment transcription: kinetics of l-strand transcription from the y-cII-oop-O-P region.

Summary Duplications of arg genes produced in the Rec⁺ and in the recA genetic backgrounds are shown by heteroduplex analysis to be strictly tandem at the level of resolution of this technique. The formation of these particular rearrangements therefore does not require the inclusion of transposons or other sequences of an appreciable size in their final structure.Duplications of short segments (about 2,000 nucleotides) appear unexpectedly stable when compared with duplications of longer segments (about 10,000 nucleotides).One of the structures analyzed displays two inversely repeated argE genes rearranged into an artificial divergent operon. The bearing of this observation on the origin of bipolar operons, of mirror-image map symmetries and on the production of inverted repeats in general, is discussed.