tapan kumar maitra explains reverse transcription, retroviruses and retrotransposons
TRANSCRIPTION generally proceeds in the direction described in the central dogma, with DNA serving as a template for RNA synthesis. In certain cases, however, the process can be reversed and RNA serves as a template for DNA synthesis. This process of reverse transcription is catalysed by the enzyme reverse transcriptase, first discovered by Howard Temin and David Baltimore in certain viruses with RNA genomes. Viruses that carry out reverse transcription are called retroviruses and examples include some important pathogens such as the Human Immunodeficiency Virus, which causes Aids, and a number of viruses that cause cancers in animals.
Retroviruses: In a typical virus particle, two copies of the RNA genome are enclosed within a protein capsid that is surrounded by a membranous envelope. Each RNA copy has a molecule of reverse transcriptase attached to it. The virus first (1) binds to the surface of the host cell and its envelope fuses with the plasma membrane, releasing the capsid and its contents into the cytoplasm. Once inside the cell, the viral reverse transcriptase (2) catalyses the synthesis of a DNA strand that is complementary to the viral RNA and then (3) catalyses the formation of a second DNA strand complementary to the first. The result is a double-stranded DNA version (4) of the viral genome. This double-stranded DNA then enters the nucleus and integrates into the host cell&’s chromosomal DNA, much as the DNA genome of a lysogenic phage integrates into the DNA of the bacterial chromosome.
Like a prophage, the integrated viral genome, called provirus, is replicated every time the cell replicates its own DNA (5). Transcription of the proviral DNA (by cellular enzymes) produces RNA transcripts that function in two ways. First, they serve as mRNA molecules (6) that direct the synthesis of viral proteins (capsid protein, envelope protein, and reverse transcriptase). Second, some of these same RNA transcripts (7) are packaged with the viral proteins into new virus particles (8). The new viruses then “bud” from the plasma membrane without necessarily killing the cell.
The ability of a retroviral genome to integrate into host cell DNA helps explain how some retroviruses can cause cancer. These viruses, called RNA tumor viruses, are of two types. Viruses of the first type carry a cancer-causing oncogene in their genomes, along with the genes coding for viral proteins. An oncogene is a mutated version of a normal cellular gene (a proto-oncogene) that codes for proteins of a cellular gene for a protein kinase. The protein product of the viral gene is hyperactive, and the cell cannot control it in the normal way. As a result, cells expressing this gene proliferate wildly, producing cancerous tumors called sarcomas. RNA tumor viruses of the second type do not themselves carry oncogenes, but integration of their genomes into the host chromosome alters the cellular DNA in such a way that a normal proto-oncogene is converted into an oncogene.
Retrotransposons: Reverse transcription also occurs in normal eukaryotic cells in the absence of viral infection. Much of it involves DNA elements called retrotransposons. Transposons are DNA segments that can move themselves from one site to another within the genome. Retrotransposons are a special class of transposon that use reverse transcription to carry out this movement. The transposition mechanism begins with transcription of the retrotransposon DNA followed by translation of the resulting RNA, which produces a protein exhibiting both reverse transcriptase and endonuclease activities. Next, the retrotransposon RNA and protein bind to chromosomal DNA at some other location site and the endonuclease cuts one of the DNA strands. The reverse transcriptase then uses the retrotransposon RNA as a template to make a DNA copy that is integrated into the target DNA site.
Although retrotransposons do not transpose themselves very frequently, they can attain very high copy numbers within a genome. Alu sequences are only 300 base pairs long and do not encode a reverse transcriptase, but using a reverse transcriptase encoded elsewhere in the genome, they have sent copies of themselves throughout the genomes of humans and other primates. The human genome contains about a million Alu sequences that together represent about 11 per cent of the total DNA. Another type of retrotransposon, called an LI element, is even more prevalent, accounting for roughly 17 per cent of human DNA. The LI retrotransposon is larger than Alu and encodes its own reverse transcriptase and endonuclease. The reason why genomes retain so many copies of retrotransposon sequences such as LI and Alu is not well understood, but they are thought to contribute to evolutionary flexibility and variability.
The writer is associate professor and head, Department of Botany, Ananda Mohan College, Kolkata