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The interaction of mRNA with proteins in vesicular stomatitis virus (VSV)-infected cells was studied by photochemical cross-linking in intact cells. The major [35S]methionine-labeled proteins which became cross-linked by UV light to mRNA in uninfected and in VSV-infected HeLa cells were similar and had apparent mobilities in sodium dodecyl sulfate-polyacrylamide gel electrophoresis corresponding to 135, 93, 72, 68, 53, 50, 43, and 36 kilodaltons. The proteins which were cross-linked in vivo specifically to the five mRNAs of VSV were labeled through radioactive nucleotides incorporated only into VSV mRNAs under conditions (5 micrograms of actinomycin D per ml) in which only VSV mRNAs are labeled. The same major mRNP proteins that became cross-linked to host mRNAs also became cross-linked to VSV mRNAs, although several quantitative differences were detected. Photochemical cross-linking and immunoblotting of cross-linked mRNPs with VSV antiserum demonstrated that in addition to host proteins VSV mRNAs also became cross-linked to the VSV-encoded N protein. The poly(A) segment of both host and VSV mRNAs was associated in vivo selectively with the 72-kilodalton polypeptide. The major proteins of mRNA-ribonucleoprotein complexes are therefore ubiquitous and common to different mRNAs. Furthermore, since the major messenger ribonucleoproteins interact also with VSV mRNAs even though these mRNAs are transcribed in the cytoplasm, it appears that nuclear transcription and nucleocytoplasmic transport are not necessary for mRNA to interact with these proteins.