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Elucidation of the mechanisms of pathogenesis underlying the diseases caused by viruses and bacteria has fascinated scientists for many years in two ways. Firstly, these pathogenic agents represent relatively sim ple biological systems for the study of basic biological processes such as replication, gene regulation, genetic variability and host-pathogen interactions. Secondly, process in this field is valuable in a practi cal sence, since it can help in the control of these diseases. The avail ability of new genetic and immunological techniques, especially recom binant DNA methods and...

The fact that none of the known DNA polymerases is able to initiate DNA chains but only to elongate from a free 3' -OH group raises the problem of how replication is initiated, both at the replication origin and on Okazaki frag- ments. It was first shown by A. KORNBERG et al. that a general mechanism to initiate replication is through the formation of an RNA primer catalyzed by RNA polymerases or by a new class of enzymes, the primases (KORNBERG 1980). This mechanism, which can be used in the case of circular DNA molecules or linear DNAs that circularize or form concatemers, cannot be used at...