Phage assembly question - Tilted Forum Project Discussion Community

clavus · 04-07-2008, 01:14 PM

Sooo how are viral phages assembled inside an infected cell? I understand that the ribosomes are hijacked into making the raw materials for the phages. But a viral phage is a complex thing. It is not just big strand of protein.

After the infected cell has made the components for the phages, do they spontaneously assemble themselves? Do they get shuttled down the ER like an assembly line while some other organelle puts them together?

How does this work? What does the work?

Martian · 04-07-2008, 04:52 PM

Well, I've already made it clear that my understanding of microbiology is minimal. But Wikipedia says:

Quote:

Originally Posted by wikipedia

n the case of the T4 phage, the construction of new virus particles involves the assistance of helper proteins. The base plates are assembled first, with the tails being built upon them afterwards. The head capsids, constructed separately, will spontaneously assemble with the tails. The DNA is packed efficiently within the heads. The whole process takes about 15 minutes.

What these helper proteins are or where they come from, I don't know.

genuinegirly · 04-07-2008, 05:28 PM

Here's what I have for you. From Molecular Genetics of Bacteria: 2nd edition by Wendy Champness, chapter 7.

Quote:

The virus injects its genes into a cell, and the cell furnishes some or all of the means to express those genes and make more viruses... To start the infection, a phage adsorbs into an actively growing bacterial cell by binding to a specific receptor on the cell surface. In hte next setp, the phage injects its entire DNA into the cell, where transcription of RNA, usually by the host RNA polymerase, begins almost immediately. However, not all the genes of a phage are transcribed into mRNA when the DNA first enters the cell. Only some of the genes of the phage have promoters that minic those of the host cell DNA and so are recognized by the host RNA polymerase. Those transcribed soon after infection are called the early genes of the phage and encode mostly enzymes involved in DNA synthesis such as DNA polymerase, primase, DNA ligase, and helicase. With the help of these enzymes, the phage DNA begins to replicate and many copies accumulate in the cell.
Next, mRNA is transcribed from the rest of the phage genes, the late genes, which may or may not be intermingled with the early genes in the phage DNA, depending on the phage. These genes have promoters that are unlike those of the host cell and so are not recognized by the host RNA polymerase alone. MOst of these genes encode proteins involved in assembly of the head and tail. After the phage particle is completed, the DNA is taken up by the heads. Finally, the cells brfeak open, or lyse, and the new phage are released to infect another sensitive cell. This whole process, known as the lytic cycle, takes less than 1 h for many phages, and hundreds of progeny phage can be produced from a single infecting phage.
Actual phage development is usually much more complex than this basic process, proceeding through several intermediate stages in which the expression of different genes is regulated by specific mechanisms. Most of the regulation is achieved by having genes be transcribed into mRNA only at certain times; this type of regulation is called transcriptional regulation...

Unfortunately most of the stuff in this chapter deals with phage gene expression and phage display procedures.

Here's a little more that's also not terribly applicable to your question:

Quote:

...Phage T4 replication occurs in two stages... In the first stage, T4 DNA replicates froma number of well-defined origins around the DNA. This type of replication is analogous to the replication of bacterial chromosomes and leads to the accumulation of single-genome-length molecules. However, these two daughter molecules will have single-stranded 3' ends because of the inability of DNA polymerase to completely replicate the ends. They lose no information, however, because the sequences at the ends of T4 DNA are repeated, i.e. are terminally redundant... Somewhat later, this type of replication ensues. The single-stranded repeated sequences at the ends of the genome-length molecules (called terminal redundancies) can invade the same sequence at the ends of other daughter DNAs, forming D-loops, which prime replication to form large branched concatemers. This replication (called recombination-dependent replication[RDR]) is analogous to the "replication restarts"... and is now known to occur in all organisms. However it was in T4 where this type of replication was first discovered... Periodic cycles of RDR lead to the synthesis of very large, branched concatemers from which individual genome-length DNAs are cut out and packaged into phage heads...

Sorry I don't have more at my disposal... maybe I'll come across it in the course of this semester. If they happen to cover it in lecture, I'll let you know. Someone has certainly studied this stuff.

clavus · 04-07-2008, 06:37 PM

Hmmm.. helper proteins.

Maybe we really don't know what does the heavy assembly work.

AKR · 04-14-2008, 07:45 PM

I remember hearing "helper proteins" somewhere in grade 11 biology class... more on this when I remember

04-07-2008, 01:14 PM	#1 (permalink)
clavus pow! Location: NorCal	Phage assembly question Sooo how are viral phages assembled inside an infected cell? I understand that the ribosomes are hijacked into making the raw materials for the phages. But a viral phage is a complex thing. It is not just big strand of protein. After the infected cell has made the components for the phages, do they spontaneously assemble themselves? Do they get shuttled down the ER like an assembly line while some other organelle puts them together? How does this work? What does the work? __________________ Ass, gas or grass. Nobody rides for free.

04-07-2008, 06:37 PM	#4 (permalink)
clavus pow! Location: NorCal	Hmmm.. helper proteins. Maybe we really don't know what does the heavy assembly work. __________________ Ass, gas or grass. Nobody rides for free.

04-14-2008, 07:45 PM	#5 (permalink)
AKR Upright	I remember hearing "helper proteins" somewhere in grade 11 biology class... more on this when I remember