DIFFERENTIAL COMPARTMENTALIZATION OF MESSENGER-RNAS IN SQUID GIANT-AXON

Previously, we reported that the squid giant axon contains a heterogen eous population of mRNAs that includes beta-actin, beta-tubulin, kines in, neurofilament proteins, and enolase. To define the absolute levels and relative distribution of these mRNAs, we have used competitive re verse transcription-PCR to quantify the levels of five mRNAs present i n the giant axon and giant fiber lobe (GFL), the location of the paren tal cell soma. In the GFL, the number of transcripts for these mRNAs v aried over a fourfold range, with beta-tubulin being the most abundant mRNA species (1.25x10(9) molecules per GFL). Based on transcript numb er, the rank order of mRNA levels in the GFL was beta-tubulin >beta-ac tin >kinesin >enolase >microtubule-associated protein (MAP) H1. In con trast, kinesin mRNA was most abundant in the axon (4.1x10(7) molecules per axon) with individual mRNA levels varying 15-fold. The rank order of mRNA levels in the axon was kinesin >beta-tubulin >MAP H1 >beta-ac tin >enolase. The relative abundance of the mRNA species in the axon d id not correlate with the size of the transcript, nor was it directly related to their corresponding levels in the GFL. Taken together, thes e findings confirm that significant amounts of mRNA are present in the giant axon and suggest that specific mRNAs are differently transporte d into the axonal domain.