Microbiology Procedure

Index >> Bacteriophages - Part Two >> Phage Genome

Phage Genome

	Phage Genome - The phage genome consists of an ssRNA strand which shows a high degree of secondary structure resulting from extensive self complementarity. The secondary structure involves 63-82% of the bases. Several hairpin structures, each consisting ora helical hydrogen bonded stem and a terminal loop of not less than three nucleotides, are found Within the stem a number of unpaired nucleotides form interior loop and bulges.
The genetic organisation of ssRNA phages is very simple, all the functions being expressed by only three genes. In the f2 group of phages the gene order appears to be as follows: 5'-A protein gene Coat protein gene Replicase gene-3'. The entire nucleotide sequence has been determined for the MS2 phage. The RNA consists of 3,569' nucleotides, Both 5' and 3' end contain extensive lengths of RNA which is not translated (129. and 174 nucleotides, respectively).
The A protein gene consists of ],179/ nucleotides. Its initiation condon is AUG and the termination codon' UAG. The coat protein gene consists of 390 nucleotides and has an AUG initiation codon and a VAA codon termination. The replicase l3 subunit gene consists of 1,635 nucleotides and has an AVO initiation codon and a U AG termination codon. Between the A protein gene1lnd the coat protein gene is an intercistronic region of 26 nucleotides. Another intercistronic region of 36 nucleotides is found between the
coat protein gene and the replicase gene. The intercistronic regions are concerned with ribosome binding and translational control mechanism. In R17 the initiation codon IS AVG for all three genes. The Q genome is as follow: 5' - A2 protein gene --AI/Coat protein gene Replicase gene-3'. The A1 and the coat protein genes have a common initiation site, but have different termination sites. The A1 polypeptide is formed when the translation of the coat protein continues beyond its termination codon
	The RNA of f2 phages is linear, and bas a sedimentation coefficient of about 27S. It contains the four bases in nearly equimolar amounts, All RNA phages have pppGp at the 5' terminus. It has been suggested that the 5' terminus forms tight hydrogen bonded loops, and thus protects the RNA molecule from exonucleolytic attacks all RNA phages the 3' end has CCA-OH, a feature also found in ay tRNAs and plant viral RNA

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