Molecular approaches to eucaryotic genetic systems / edited by Gary Wilcox, John Abelson, C. Fred Fox.

Date:
1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Molecular approaches to eucaryotic genetic systems / edited by Gary Wilcox, John Abelson, C. Fred Fox. Source: Wellcome Collection.

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    8 1. JOHN С. FIDDES amount of DNA though the existence of overlapping genes in such systems cannot be ruled out. The nucleotide sequence of 0X174 is now stored on a computer and this has facilitated the searches for particular sequences (R. Staden and B.G. Barrell, personal communication). One interesting feature which has emerged from this study is that 0X174 appears to have the potential for other systems of overlapping genes. Thus a total of 15 A-T-G codons, which are preceded by sequences with characteristics of ribosome binding sites, have been identified. Proteins, ranging in size from 22 amino acids long to one example in gene H which would be 191 or 201 amino acids long, could conceivably be synthesised from these initiation codons. This observation probably indicates that the full nucleo¬ tide sequence requirements for ribosome binding are not under¬ stood and that these extra sites are not used. There are, however, indications that 0X174 has extra genes. SDS Poly¬ acrylamide gels of 0X174 infected cells show minor reproduc¬ ible bands which have not been identified (13). Also there are several viral induced functions which have not yet been correlated with the known viral proteins. For example, on infection 0X174 induces a methylase which introduces a single 5-methyl cytosine into the viral DNA (33) and a phage function is responsible for the shut off of host cell DNA synthesis, though this may be attributed to A, the translational, in phase, restart of gene A (34). Also the proteins which are responsible for sizing unit length viral DNA and ligating it have not been unequivocally determined. Intercistronic sequences. The three genes of the RNA phage such as MS2 and Qß are separated by two distinct inter¬ cistronic regions which range in size from about 25 to 40 nucleotides (35). With 0X174 the situation is not so straight¬ forward due to the existence of overlapping initiation and termination codons. Between the 5' termini of genes A and J in 0X174, a region which encompasses 42% of the phage genome, there is no untranslated region since the initiators and terminators of genes A and C, С and D, and D and J overlap (see Fig. 1). The extent of the С coding region has recently been established (B.C. Barrell, unpublished results) from the sequence analysis of a С mutant, ochó. These three examples of overlapping initiators and termi¬ nators in 0X174 are shown in Fig. 5. Two types of overlapping codons are found - г 1 ( AiT-G .A I and ' T-AjAiT-G J I J though it may be significant that both result in the two genes having different reading phases, A similar example has been