Genes Dev.2: [863][864][865][866][867][868][869][870][871][872][873] 1988). We have purified a 59.5-kDa nuclear protein (suGF1) from sea urchin embryos by DNA affinity chromatography. suGF1 has high binding affinity and specificity for oligo(dG) -oligo(dC). The identity of the purified protein was confirmed by renaturation of sequence-specific DNA-binding activity from a sodium dodecyl sulfate-polyacrylamide gel slice and by Southwestern (DNA-protein) (4,19). In addition, G-string-binding factors have been detected in a number of different tissues from various organisms (8,19,22,38). The promoter of the chicken adult 3-globin gene contains a G string of 16 to 18 Gs (22, 28). There is strong evidence that regulation of expression of this gene involves a complex interplay between binding of a G-string factor, DNA conformation, and displacement of a positioned nucleosome at the G string (22). A G6 string has also been implicated in regulation of gene expression during sea urchin development via binding of a G-string factor to a cis-regulatory element present in the promoter of a cell lineage-specific gene LpS13 (38). This sea urchin G-string factor may be related to a putative mammalian transcription factor, IF1, implicated in the coordinate regulation of expression of the (xl (I) and a2 (I) collagen genes during embryonic development via binding to G7 strings present in both promoters (17, 38). Interestingly, a G7 string occurs within a purine-rich region of the chicken ct2(I) collagen gene promoter which is hypersensitive to Si nuclease and DNase I in chromatin in a tissuespecific fashion (24).G strings have in common with homopurine-homopyrimidine (pur. pyr) stretches the ability to form unusual DNA structures such as triple helices in vitro, the formation of which depends critically on the degree of superhelical stress of the DNA, the length of the homopurine stretch, and the chemical environment (21,36). Several authors have proposed that G strings may function in vivo as a conformational switch which is modulated in some way by G-stringbinding proteins (4,19,20,24).A similar role has been proposed for G-rich regions which occur within pur. pyr stretches upstream of several housekeeping genes such as the c-Ki-ras, epidermal growth factor receptor, and c-myc genes (6,12,14,23,29). These pur-pyr regions are S1 nuclease sensitive in vitro and bind to factors containing multiple copies of the sequence GGGNGGG in their DNA recognition sites. In some cases, alterations in chromatin structure correlating with the state of expression of these genes have been mapped to the pur. pyr regions in nuclei.These observations raise several unresolved issues. G strings may play a role in gene regulation during development via families of G-string factors with related functions. The structural and functional relationships among various G-string factors as well as factors binding to G-rich sequences within pur-pyr stretches need to be examined, considering the parallels that can be drawn between their proposed biological role...