Transcription in vivo of the late genes of bacteriophage T7 produces 12-13 messenger RNAs. Our objective is to map the transcription units producing these mRNAs and to determine the location of initiation, termination, and RNase III cleavage sites on the T7 genome. The proteins encoded in the mRNAs will be determined after separating the RNA species by a newly developed preparative polyacrylamide gel electrophoresis apparatus. The separated RNAs will be translated in a cell-free protein-synthesizing system and the radioactive polypeptides will be analyzed by sodium dodecyl sulfate polyacrylamide gel in the presence and absence of urea, and also by two-dimensional isoelectric focusing and electrophoresis. Translational control of T7 gene expression will be studied at two levels. The first is the apparent funtional inactivation of chemically stable T7 early mRNA. Specific early messages will be purified and their mRNA template activity will be determined early and late in infection, in the presence and absence of T7 late mRNAs. The functional inactivation of a late mRNA physically attached to a polycistronic early mRNA will also be studied to test the specificity of functional inactivation. The second area of translational control studies will be the regulation of the level of late proteins synthesized from the three gene 10 mRNAs. The organization of transcription units on the bacteriophage T4 genome is now amenable to the same approach described for T7, since active mRNAs coding for high molecular weight proteins can be isolated. The size of specific mRNAs representing immediate-early, delayed-early, quasi-late and late genes will be determined by mRNA fractionation on preparative polyacrylamide gels followed by translation in vitro. The possible co-transcription of immediate-early and delayed-early genes in particular will be studied. The study will also reveal whether some quasi-late genes, such as genes 43 and rIIB, are transcribed from two promotors.