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Index >> Bacteriophages - Part One >> Protein Synthesis of Phage T4

Protein Synthesis of Phage T4

	Protein Synthesis of Phage T4 - The phage utilizes the host protein synthesis machinery(ribosomes, RNA polymerase. transcription systems) to synthesize its proteins. Its genome contains early genes arid late genes which transcribe at different times after infection. Experimental evidence indicates that there are two general classes of phage messenger RNAs, early messengers and fate messengers. Early messengers are present 5 minutes after infection, before replication begins (at 6 minutes). Late messenger synthesis begins at about 10 minutes.
At 20 minutes both early and late messengers are present, but the average concentration of early messengers is lower than at 5 minutes. T4 early and late messengers appear to be transcribed mostly from different strands of DNA. The early genes are transcribed from the 1 strand and the late genes largely from the r strand.
Early messengers are of two classes. The immediate early messengers are the sequences transcribed during the first 1.25 or 2.5 minutes after infection. RNA can be transcribed in the presence of chloramphenicol Delayed early messengers are the sequences expressed during 2.5-3.75 minutes. The synthesis reaches a peak by 5 minutes. Delayed early messenger synthesis is prevented by chloramphenicol. The late messengers include the quasi late messengers and the true late messengers. Salser, Bolle and Epstein (1970) have described the quasi late messengers.
These sequences are common to both early and late populations. They are present at low concentration at 5 minutes, and at high concentration at 20 minutes. True late messengers are synthesized from about 11 minutes. They are present in the 20 minute RNA population, but are effectively absent from the 5 minute population.
	The early transcripts include antilate messengers. They form RNA-RNA duplexes on annealing with the 20 minute RNA, and are therefore complementary to it. The annealing reaction is the result of hybridization between lat: r strand sequences and antilate 1 strand sequences. The antilate messengers do not appear to be translated into protein, and their function is not known. The early transcribed genes include some for DNases which recognize base sequences containing clusters of cytosine in host 'DNA, but do not affect the HMC: containing phage DNA. The host DNA thus ceases to function, enabling increased functioning of viral genes. Moreover, the nucleotides made available by breakdown of host DNA are used for synthesizing progeny phage particles. Thus the phage inhibits or destroys host functioning which might hinder its own multiplication. T4 infected bacterial cells rapidly synthesize a phage coded enzyme which dephosphorylates dCTP and dCDP, and removes them from the nucleotide pool. This eliminates competition between cytidine triphosphate (dCTP) and hydroxymethyicytidine triphosphate (HMgCTP) for incorporation into the growing phage DNA chain, and dCTP is not incorporated into phage DNA.

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