RESTRICTED BETA-GALACTOSIDASE EXPRESSION OF A HYGROMYCIN-LACZ GENE TARGETED TO THE BETA-ACTIN LOCUS AND EMBRYONIC LETHALITY OF BETA-ACTIN MUTANT MICE

beta-actin is a cytoskeletal protein that is ubiquitously expressed. T o exploit the regulation the beta-actin gene, a promoterless hygromyci n-lacZ fusion gene with a splice acceptor was introduced into the firs t intron of the beta-actin locus by homologous recombination in mouse embryonic stem (ES) cells. The targeted ES cells were hygromycin resis tant and expressed beta-galactosidase (beta-gal) activity. However, no beta-gal activity was detected in heterozygous embryos. In adult hete rozygotes, beta-gal activity was detected only in testes. RT-PCR analy sis demonstrated the presence of both beta-actin exon 1-hygromycin- an d exon 1-exon 2-containing transcripts in homozygous mutant embryos. L acZ-containing transcripts were detected in adult heterozygous testes and, surprisingly, in homozygous mutant embryos. These results demonst rate that the integration of the hygromycin-lac Z gene into the first intron of the beta-actin locus was not productive for the ubiquitous e xpression of activity. Because this integration mimics beta-gal certai n types of gene trap events, it suggests that caution should be used w hen interpreting beta-gal expression patterns in genetic screens using gene trap strategies. In addition, mice homozygous for the beta-actin mutation developed normally up to embryonic day 8.5 (E8.5) but became growth retarded at E9.5 and subsequently died. The RT-PCR data indica te that this targeted mutation is a hypomorphic allele of beta-actin.