The carboxy-terminal domain (CTD) of the large subunit of RNA polymerase II is essential in vivo, and is found in either an unphosphorylated (IIa) or hyperphosphorylated (IIo) form. The Drosophila uninduced hsp70 and hsp26 genes, and the constitutively expressed beta-1 tubulin and Gapdh-2 genes, contain an RNA polymerase II complex which pauses after synthesizing a short transcript. We report here that, using an in vivo ultraviolet crosslinking technique and antibodies directed against the IIa and IIo forms of the CTD, these paused polymerases have an unphosphorylated CTD. For genes containing a 5' paused polymerase, passage of the paused RNA polymerase into an elongationally competent mode in vivo coincides with phosphorylation of the CTD. Also, the level of phosphorylation of the CTD of elongating polymerases is shown not to be related to the level of transcription, but is promoter specific.

To investigate functional differences between RNA polymerases IIA and II0 (Pol IIA and Pol II0), with hypoand hyperphosphorylated carboxy-terminal repeat domains (CTDs), respectively, we have visualized the in vivo distributions of the differentially phosphorylated forms of Pol II on Drosophila polytene chromosomes.Using phosphorylation state-sensitive antibodies and immunofluorescence microscopy with digital imaging, we find Pol IIA and Pol II0 arrayed in markedly different, locus-and condition-specific patterns. Major ecdysone-induced puffs, for example, stain exclusively for Pol II0, indicating that hyperphosphorylated Pol II is the transcriptionally active form of the enzyme on these genes. In striking contrast, induced heat shock puffs stain strongly for both Pol IIA and Pol II0, suggesting that heat shock genes are transcribed by a mixture of hypo-and hyperphosphorylated forms of Pol II. At the insertion sites of a transposon carrying a hybrid hsp70-lacZ transgene, we observe only Pol IIA before heat shock induction, consistent with the idea that Pol II arrested on the hsp70 gene is form IIA. After a 90-sec heat shock, we detect heat shock factor (HSF) at the transposon insertion sites; and after a 5-min shock its spatial distribution on the induced transgene puffs is clearly resolved from that of Pol II. Finally, using antibodies to hnRNP proteins and splicing components, we have discerned an apparent overall correlation between the presence and processing of nascent transcripts and the presence of Pol II0.[Key Words: RNA polymerase IIA/II0; CTD phosphorylation; transcription factors; in situ localization; immunofluorescence microscopy; digital imaging] Received July 28, 1993; revised version accepted September 29, 1993.The carboxy-terminal repeat domain (CTD) of the largest subunit of RNA polymerase II (Pol II) is an unusual entity composed of multiple repeats of the 7-amino-acid consensus sequence YSPTSPS (for review, see Corden 1990;Young 1991). The number of repeats ranges from 26 in yeast, to 42 in Drosophila, to 52 in mammals. The CTD has been shown to carry out essential in vivo roles in yeast (Nonet et al. 1987), Drosophila (Zehring et al. 1988), and mammalian cells (Bartolomei et al. 1988), but precisely what those roles are is not well understood. Suggested roles for the CTD include interacting with transcription initiation factors, serving as a molecular "cowcatcher" to facilitate movement of polymerase on chromatin templates, providing a link between transcription and RNA processing, and localizing polymerase to specific nuclear compartments (Corden 1990 (Allison et al. 1988;Scafe et al. 1990;Peterson et al. 1991), suggesting that the CTD may be involved in receiving regulatory signals at certain promoters. In vivo and in vitro experiments suggest that these responses might be mediated through the TATA-binding protein (TBP) component of TFIID (Koleske et al. 1992;Usheva et al. 1992;Thompson et al. 1993). On the other hand, the precise nature of the CTD involvement in initiation is not known, ...

Saccharomyces cerevisiae CTDK-I is a protein kinase complex that specifically and efficiently hyperphosphorylates the carboxyl-terminal repeat domain (CTD) of RNA polymerase II and is composed of three subunits of 58, 38, and 32 kDa. The kinase is essential in vivo for normal phosphorylation of the CTD and for normal growth and differentiation. We have now cloned the genes for the two smaller kinase subunits, CTK2 and CTK3, and found that they form a unique, divergent cyclin-cyclin-dependent kinase complex with the previously characterized largest subunit protein CTK1, a cyclin-dependent kinase homolog. The CTK2 gene encodes a cyclin-related protein with limited homology to cyclin C, while CTK3 shows no similarity to other known proteins. Copurification of the three gene products with each other and CTDK-I activity by means of conventional chromatography and antibody affinity columns has verified their participation in the complex in vitro. In addition, null mutations of each of the genes and all combinations thereof conferred very similar growth-impaired, cold-sensitive phenotypes, consistent with their involvement in the same function in vivo. These characterizations and the availability of all of the genes encoding CTDK-I and reagents derivable from them will facilitate investigations into CTD phosphorylation and its functional consequences both in vivo and in vitro.