|John Guest FRS|
Studying the genes and enzymes required to synthesise the amino acid Tryptophan in E.coli, may seem a little esoteric, but this field has given us many applications in Biotechnology, and arguably the work that John published during his time in Charles Yanofsky's laboratories ranks amongst the most significant in the field of Molecular Biology. The key paper can be found at this link, where the full text is free to download and read! You wont find an extensive page on wikipedia about this work, but I believe the experiments published between 1964 and 1967 are fundamental to our understanding of the nexus of nucleotide chemistry and protein chemistry in the evolution of life. It is also important to mention the Nobel Prize winning work discussed by Rich Roberts in his guest blog, that led Roberts and Sharp to independently demonstrate that there is an interruption in this colinear relationship between nucleic acid coding information and the primary structure (amino acid sequence) in higher organisms. These interruptions give rise to exons (coding sequence) and intervening sequences (introns). In fact only 10% of our own genome appears to code for proteins or RNA molecules.
Back to some chemistry and the fact that in the 1960s it was immensely challenging to obtain the sequence of a protein: no mass spectrometry in those days: in fact John and his colleagues would have to rely on a semi manual approaches that were developed by pioneering protein chemsits including Stein and Moore in the USA and Fred Sanger in the UK, which involved complex analysis of proteolytic fragments (see RHS). Another important piece of work carried out in the laboratory of Vernon Ingram using this powerful technique of protein fingerprinting was to establish the molecular basis of sickle cell anaemia in haemoglobin. Worse still it would be another 10 years at least before DNA sequencing would become possible (thanks in part to Fred Sanger again!), so the team had to use recombination frequencies in order to match the gene and the protein sequences. All in all, this was an experimental and intellectual tour de force and it came along as the genetic code was cracked and a few years before Masayasu Nomura reconstituted the bacterial ribosome: the site of protein synthesis. The complete set of papers published by John in while he worked in the Yonofsky lab can be found here. I recommend those of you interested in the intellectual development of modern molecular biology take a look.
John kindly emailed me an example of his original peptide fingerprinting data from his time in Yanofsky's lab. It is shown below.
I dont think you need to be an expert in protein chemistry to see that this experiment reveals a significant difference in the peptide patterns (purple spots) between the wild type protein (left) compared with the mutant (right). This is a lovely example of how a carefully planned and executed experiment can be so revealing. John has commented on the Blog below.
The story doesn't end here though. Placing this work in context is important. Previous landmark experiments had demonstrated the relationship between genes and proteins: the classic work of Beadle and Tatum on Neurospora had laid the main foundations. Crick, Brenner, Khorana (a PhD graduate of the University of Liverpool), Ochoa, Nirenberg and collaborators (see Wiki site) were all making significant progress on the determination of the Genetic Code: the relationship between the nucleotide sequences in mRNA (and the role of tRNA) in defining the sequence of amino acids in a protein chain. By the end of the 1960s, a robust model existed for the bacterial processes of replication, transcription and translation, the mechanisms underlying the central dogma were well established: DNA makes RNA makes protein, and John was a major contributor. By the end of the 1970s, there were discrepancies emerging in respect of higher organisms: Roberts and Sharp had discovered the phenomenon of splicing, in which the message (mRNA) is first processed into a mature form before the process of translation can begin. Moreover, another 10 years later, it was observed (particularly in ancient bacteria) that protein sequences are sometimes spliced! It had been discovered that some proteins lose their ends before they become fully functional, but the discovery of a mechanism in which internal sequences of protein are chopped out and the chain re-joined, was unexpected. In fact many of the DNA Polymerases used in PCR are processed in this way. The excised segments are called inteins (and the protein equivalent of exons are known as exteins). We also now know that some sequences of DNA can encode more than one protein, achieved by starting translation in a different "reading frame" or register. In order to confound us further some organisms (such as protozoa) use a normal stop codon to encode the aromatic amino acid Tryptophan (bringing us full circle to the Trp operon work in Yanofsky's lab!).
Despite these anomalies, we still observe the main findings that John made in Charles Yanofsky's lab hold true for the vast majority of gene:protein relationships, with the caveat that splicing is generally required in higher organisms. When you carry out a BLAST search to compare your sequences, it wouldn't work if we didn't have a robust understanding of the Genetic Code and the co-linearity of the gene and protein. I hope you agree with me that the experiments of the pioneering molecular biologists after the Watson and Crick discovery represent some of the most impressive in all of Science.