Restriction Enzymes: The Key to DNA Mystery

`The book of life' is how the DNA molecule is often referred to, and this analogy makes it possible to understand why the DNA is one of the prime targets of research in modern biology. To elucidate the analogy, the `book' consists of long sequences, or `sentences', with individual genes, or `words', which are in turn made of base pairs A, T, G, C, which are the basic `alphabets'! If we could but read the `book' in detail, the secret of life would be revealed, and many of life's processes could be controlled.

In recombinant DNA research, DNA fragments from different origins can be joined; the genes on these fragments can then be located, and analysed. By turning on the genes, large amounts of useful proteins can be made.

Recombinant DNA research can thus be likened to the `find', `cut' and `paste' functions in a word processor. But, we need to know what agent or tool is available to cut the long DNA sequences into defined pieces, before specific genes can be identified by other means. Studies show that it is the restriction enzymes that could do the job, i.e. they are the scissors in DNA research.

What are restriction enzymes? They are bacterial enzymes that can not only recognize, but also cleave specific DNA sequences. The cleaved DNA hence have defined ends and can be joined on to another DNA fragment having similar ends. The repertoire of restriction enzymes, as it exists, can only recognize about 200 specific cleavage sites, and many DNA sequences are not covered by this. The current technology is hence much like an incomplete word processor that can only recognize a limited number of sites on the sentences. So, what happens to the unrecognized sites? And, to make matters even more difficult, some of these restriction enzymes are rather rare, and not commercially available.

In an attempt to overcome these limitations, a project was initiated by Dr. P. C. Shaw of the Department of Biochemistry. The aim was to try and find novel or rare restriction enzymes from bacterial strains in Hong Kong and its neighbouring regions. This research on restriction enzymes was supported initially by two direct grants from the Research Grants Council, and subsequently, by a contract grant from a biochemical company in the USA and a research grant from the Croucher Foundation.

Until now, more than 100 restriction enzymes have been found in Dr. Shaw's laboratory. Some of these are unique or rare enzymes, or possess properties superior to previously known ones. The findings have been published in prestgious journals, including Nucleic Acids Research and Gene. Some of these strains, proudly carrying the designation HK for Hong Kong — for example, the BsiHKAI and the EclHKI — have made their way to commercial production by biochemical companies in the USA, with transfer fees and royalties accruing to the University.

After these restriction enzymes were identified, they have been studied and some very interesting properties discovered. In a bacterial cell, for instance, methyltransferase coexists with restriction enzyme and the two of them form a restriction-modification system. Methyltransferase modifies the bacterial DNA to prevent the unwanted cleavage of its own restriction enzyme. In analysing the restriction-modification system of one of the clinically-isolated strain Escherichia coli HK31, Dr. Shaw's group has discovered an unusual methyltransferase M.EcoHK31I which requires two proteins instead of the usual one for functioning. Dr. Richard Roberts, the 1993 Nobel laureate in physiology or medicine, has mentioned this discovery in the Federation of American Societies for Experimental Biology Summer Research Conference on Restriction Endonucleases and Methyltransferases held at Vermont, USA, in 1993, and has subsequently reported it in Volume 5 Issue 1 of the NEB Transcript. The genes encoding the above restriction-modification system are now being further analysed.