Lambda’s Relatives and Lambda Hybrids
Lambda does not uniquely occupy its ecological niche. A variety of both close and distant relatives of lambda are known. The near relatives all possess the same sticky ends of the chromosome and are of almost the same size and genetic structure as lambda. Some possess the same immunity as lambda, which means that they cannot grow in lambda lysogens, but others possess different immunities and can grow in lambda lysogens (Table 14.1). That is, they are heteroimmune. Remark-ably, lambda relatives can form recombinants between one another to form hybrids. DNA heteroduplexes between them show that their genes tend to be either closely homologous or quite dissimilar. It is as though nature possesses a few fundamental lambda-type phage and can inter-change their parts to produce the large number of different lambdoid phage that are observed.
The regions of the phage λ genome that are replaced by segments of 21 and 434 in hybrid λimm21 and λimm434.
A study of the similarities and differences between lambda’s relatives has been fruitful in understanding lambda. Phage 21, φ80, and 434 have been particularly useful in the research on phage lambda. They have highlighted the crucial invariants of lambdoid growth and regulation. Recombinant hybrids have also been constructed between lambda and both 21 and 434 (Fig. 14.3). These were constructed so as to retain the immunity of the heteroimmune phage but to contain as much of the rest of lambda as possible. These lambda hybrids are called λimm434, or λi434, and λi21. They permit recombination and complementation stud-ies that otherwise would be impossible because of the existence of immunity.
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