Recombinant phages between T7 and T3 have been isolated that grow well on strains of Escherichia coli that contain the F factor. One phage that has been characterized physically and genetically is predominately of the T7 genotype. Within this hybrid phage, two separate regions of T3 DNA have been located which are necessary for the phenotype of productive growth on F-containing strains. One of these, designated Ml, contains the right part of gene 1 and continues through gene 1.3; the second, gene 4.

The in vivo function of the gene 2 protein of bacteriophage T7 has been examined. The gene 2 protein appears to modulate the activity of the gene 3 endonuclease in order to prevent the premature degradation of any newly-formed DNA concatemers. This modulation is not however a direct interacton between the two proteins. In single-burst experiments rifamycin can substitute for the gene 2 protein, allowing formation of fast-sedimenting replicative DNA intermediates and progeny phage production. This suggests that the sole function of the gene 2 protein is inhibition of the host RNA polymerase and that the latter enzyme directs or promotes the endonucleolytic action of the gene 3 protein.

We have constructed a hybrid transcriptional unit, designated SBL, in which the enhancer-containing regulatory regions from SV40 and BK virus were inserted, in tandem, upstream of the adenovirus major late promoter. This hybrid promoter was used to construct a eucaryotic shuttle vector, pSBL, comprised of the ampr gene from pBR322, the SBL promoter, and 3' regulatory sequences of SV40 containing the small t splice site and polyadenylation signals. The hybrid promoter can be excised from pSBL on a Pvu.II cassette. A unique Bcl.I site was included to allow any gene of interest to be inserted downstream of the SBL promoter. To assess the strength and utility of the SBL promoter, a chloramphenicol acetyltransferase (CAT) expression vector, pSBL-CAT, was constructed. Following transfection of this expression vector into a variety of mammalian host cells, the level of CAT activity was measured 48 to 72 h later as described previously (1). The level of CAT activity obtained from pSV2-CAT, in which the CAT gene is driven by the strong SV40 early promoter (1), was used for comparative purposes. The SBL promoter was 3 to 6 fold stronger than the SV40 early promoter in the following cell lines: BHK-21, HeLa, MK2, COS-1, 293, CHO (all available from the American Type Culture collection), UCLA-P3 (2), K816 (3), and an adenovirus-

The T7-T3 recombinant phage B02, whose isolation and physical properties are described in the accompanying paper (Molineux et al., J. Virol. 46:881-894, 1983), has been characterized by physiological means after infection of male (F plasmid-containing) and female strains of Escherichia coli. Single-step growth analyses have shown that the hybrid phage gives a burst about two-thirds that of T7 or T3 on females and about one-half that of T3 on males. This reduced burst size can be correlated with altered kinetics of macromolecular synthesis, probably at the level of transcription. The T3 insertions of B02, designated Ml and M2, that are essential for growth of the hybrid phage on male strains, have been shown to be trans acting and can rescue T7 from F-mediated restriction. The nature of the gene products encoded by the Ml and M2 regions and their role in overcoming the abortive infection of males by T7 are discussed.

Heterogeneity of the 5‐hydroxytryptamine2 (5‐HT2) receptor across species has been implicated in several pharmacological and physiological studies. Although 5‐HT2 receptors in the rat have been linked to increases in Phosphoinositide (PI) hydrolysis, little evidence exists to support the association of guinea pig 5‐HT2 receptors with Pl hydrolysis, the second messenger generally linked with 5‐HT2receptors. In the present study, we have taken a molecular and biochemical approach to determining whether species differences in brain 5‐HT2 receptors exist between rat and guinea pig. First, we isolated partial cortical 5‐HTa receptor cDNA clones that encompassed the third intracellular loop, a receptor area putatively important in receptor‐effector coupling. The amino acid sequences deduced from the cDNA clones for rat and guinea pig brain 5‐HT2 receptor were 97% homologous. However, the guinea pig 5‐HT2 receptor had two tandem substitutions that disrupted a potential alpha helix in the region of the third cytoplasmic loop, which theoretically could alter the intracellular coupling of the guinea pig cortical 5‐HT2 receptor. Because of these molecular differences, we examined further the pharmacological activation of the brain 5‐HT2 receptor from guinea pig. 5‐HT and the 5‐HT2 receptor agonist α‐methyl‐5‐HT increased PI hydrolysis in guinea pig cortical slices whereas the 5‐HT1c receptor agonist 5‐methyltryptamine was significantly less potent. In addition, the 5‐HT2 receptor antagonists LY53857, ketanserin, and spiperone blocked 5‐HT‐stimulated Pl hydrolysis. These pharmacological data suggested that activation of the 5‐HT2 receptor in guinea pig cortical slices was associated with PI hydrolysis. Thus, although areas of the guinea pig brain 5‐HT2 receptor that influence receptor‐effector coupling were different from the rat, such differences were not critical to receptor‐effector coupling because, as in the rat, guinea pig brain 5‐HT2 receptors were also coupled to PI hydrolysis.