DNA-binding proteins on lampbrush chromosome loops.

When fixed newt lampbrush chromosomes are treated with RNase to remove nascent transcripts and are then probed with radiolabeled single-stranded DNA in 0.1 x SSC, proteins associated with the majority of the lateral loops bind the probe nonspecifically. One or more common hnRNP proteins, several of which are known to bind single-stranded DNA, could be responsible for this generalized binding. In 1.0 x SSC only a relatively small subset of loops continues to bind the probe. In order to characterize this subset of loops, we prepared polyclonal antibodies against DNA-binding proteins initially identified by "Southwestern" analysis. We show by an in situ double labeling experiment that a polyclonal serum raised against gel-eluted histone H1 recognizes the same lateral loops that bind DNA in 1.0 x SSC.