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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    10 1. JOHN С. FroOES does represent this terminator then its position immediately after the promoter would appear strange since it would presumably result in premature termination of transcrip¬ tion. However, it has been proposed that part of the recogni¬ tion signal for transcriptional termination is a hairpin loop structure in the mRNA (Rosenberg et al.» unpublished, and 39). RNA species synthesised from positions before the A promoter would have such a secondary structure (Fig. 2) and might therefore be expected to terminate at the sequence ТбА. How¬ ever, mRNA which was initiated at the A promoter would not have this structure and would therefore not be subject to termination. The secondary structure between genes J and F (Fig. 6) may also be involved in the termination of transcription since this site has been associated with one of the weak transcrip¬ tional terminators of 0X174. There is, however, no sequence of the form T5_5A. There is no evidence for a promoter in this region. The largest untranslated space in 0X174 is the 111 nucleotide gap between genes G and F (40). There is evidence to substantiate the existence in 0X174 viral DNA of both the hairpin loops in this region (Fig. 6). The depurination products obtained from the viral DNA fragment resistant to the action of the single strand specific nucleases of N. crassa (41) match those predicted from the nucleotide sequence of the major loop. The depurination products from a similar fragment isolated through its resistance to the enzyme (H. Schaller, personal communication) indicates that both of the loops shown in Fig. 6 are found. The function of this region is not known. The loop structures may represent the weak transcriptional termination site observed between genes F and G or they may be involved in the binding of a specific viral protein in the processes of assembly or penetration. Alternatively the function of this region may be in the double stranded form of the DNA since the symmetrical region formed by the loop has an unusual base distribution with a central run of 17 adjacent A-T base pairs flanked by regions in which seven out of nine and 10 out of 12 base pairs are G-C. Sequences which have both a control and a coding function. In several regions of the 0X174 genome the DNA sequence has both a coding and a control function. The most striking example of this economy of DNA usage is shown by the sequence which includes the 3' termini of genes D and E and the 5' terminus of gene J (see Fig. 7). In this region the nucleotide sequence 5' A-A-G-G-A-G-T- G-A-T 3' has three functions. With one nucleotide looped out