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Over 50 years of biological research with bacteriophage T4 includes notable discoveries in post-transcriptional control, including the genetic code, mRNA, and tRNA; the very foundations of molecular biology. In this review we compile the past 10 - 15 year literature on RNA-protein interactions with T4 and some of its related phages, with particular focus on advances in mRNA decay and processing, and on translational repression. Binding of T4 proteins RegB, RegA, gp32 and gp43 to their cognate target RNAs has been characterized. For several of these, further study is needed for an atomic-level perspective, where resolved structures of RNA-protein complexes are awaiting investigation. Other features of post-transcriptional control are also summarized. These include: RNA structure at translation initiation regions that either inhibit or promote translation initiation; programmed translational bypassing, where T4 orchestrates ribosome bypass of a 50 nucleotide mRNA sequence; phage exclusion systems that involve T4-mediated activation of a latent endoribonuclease (PrrC) and cofactor-assisted activation of EF-Tu proteolysis (Gol-Lit); and potentially important findings on ADP-ribosylation (by Alt and Mod enzymes) of ribosome-associated proteins that might broadly impact protein synthesis in the infected cell. Many of these problems can continue to be addressed with T4, whereas the growing database of T4-related phage genome sequences provides new resources and potentially new phage-host systems to extend the work into a broader biological, evolutionary context.
Bacteriophage T4 multiples poorly in Escherichia coli strains carrying the defective prophage, e14; the e14 prophage contains the lit gene for late inhibitor of T4 in E. coli. The exclusion is caused by the interaction of the e14-encoded protein, Lit, with a short RNA or polypeptide sequence encoded by gol from within the major head protein gene of T4. The interaction between Lit and the gol product causes a severe inhibition of all translation and prevents the transcription of genes downstream of the gol site in the same transcription unit. However, it does not inhibit most transcription, nor does it inhibit replication or affect intracellular levels of ATP. Here we show that the interaction of gol with Lit causes the cleavage of translation elongation factor Tu (EF-Tu) in a region highly conserved from bacteria to humans. The depletion of EF-Tu is at least partly responsible for the inhibition of translation and the phage exclusion. The only other phage-exclusion system to be understood in any detail also attacks a highly conserved cellular component, suggesting that phage-exclusion systems may yield important reagents for studying cellular processes.
Lit was first identified in 1989 :ref{doi=10.1128/jb.169.3.1232-1238.1987}, stands for late inhibitors of T4, and was found to inhibit phage T4 in Escherichia coli (K12). The Lit gene is found in the e14 cryptic prophage :ref{doi=10.1128/jb.170.5.2056-2062.1988}. Lit is also partially active against other T-even phages :ref{doi=10.1073/pnas.91.2.802}.
The Lit system detects cleaves EF-Tu translation factor :ref{doi=10.1073/pnas.91.2.802} at a late stage of phage maturation, when the major capsid protein binds to EF-Tu and triggers its cleavage by Lit :ref{doi=10.1074/jbc.M002546200}. As a result, the translation is inhbited, which ultimately leads to cell death. Lit is part of the abortive infection category of defense systems.
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