THE PROXIMAL SEQUENCE ELEMENT (PSE) PLAYS A MAJOR ROLE IN ESTABLISHING THE RNA-POLYMERASE SPECIFICITY OF DROSOPHILA U-SNRNA GENES

Most small nuclear RNA (snRNA) genes are transcribed by RNA polymerase II, but some (e.g., U6) are transcribed by RNA polymerase iii, In ver tebrates a TATA box at a fixed distance downstream of the proximal seq uence element (PSE)acts as a dominant determinant for recruiting RNA p olymerase III to U6 gene promoters. in contrast, vertebrate snRNA gene s that contain a PSE but lack a TATA box are transcribed by RNA polyme rase II, In plants, transcription of both classes of snRNA genes requi res a TATA box in addition to an upstream sequence element (USE), and polymerase specificity is determined by the spacing between these two core promoter elements, in these examples, the PSE (or USE) is Interch angeable between the two classes of snRNA genes, Here we report the su rprising finding that the Drosophila U1 and U6 PSEs cannot functionall y substitute for each other; rather, determination of RNA polymerase s pecificity is an intrinsic property of:he PSE sequence itself, The alt eration of two or three base pairs near the 3'-end of the U1 and US PS Es;was sufficient to switch the RNA polymerase specificity of Drosophi la snRNA promoters in vitro, These findings reveal a novel mechanism f or achieving RNA polymerase specificity at insect snRNA promoters.