The effects of an interactive book reading program were assessed with children from low-income families who attended subsidized day-care centers in New York. The children entered the program with language development in standard English vocabulary and expression that was about 10 months behind chronological age on standardized tests. Children were pretested and assigned randomly within classrooms to 1 of 3 conditions: (a) a school plus home condition in which the children were read to by their teachers and their parents, (b) a school condition in which children were read to only by teachers, and (c) a control condition in which children engaged in play activities under the supervision of their teachers. Training of adult readers was based on a self-instructional video. The intervention lasted for 6 weeks, at which point children were posttested on several standardized measures oflanguage ability that had been used as pretests. These assessments were repeated at a 6month follow-up. Educationally and statistically significant effects of the reading intervention were obtained at posttest and follow-up on measures of expressive vocabulary.According to the 1991 Carnegie Foundation for the Advancement of Teaching report, Ready to Learn: A Mandate for the Nation, 35% of children in the United States enter kindergarten unprepared to learn, with most lacking the vocabulary and sentence structure crucial to school success. Although there are some problems with the methods of this report and inherent difficulties in dichotomizing school readiness, there is no doubt that there are very large individual differences in early educational achievement that have long-term consequences for children and society (Alexander & Entwisle, 1988;Stevenson & Newman, 1986).Why are so many children, particularly those from low-income families, deficient in the skills that are critical to school readiness? Children's preschool experience with books may play an important role. Adams (1990, p. 85) estimated that a typical middle-class child enters first grade with 1,000 to 1,700 hr of one-on-one picture book reading, whereas the correspond-

Bacteriophage ϕC31 encodes an integrase, which acts on the phage and host attachment sites, attP and attB, to form an integrated prophage flanked by attL and attR. In the absence of accessory factors, ϕC31 integrase cannot catalyse attL x attR recombination to excise the prophage. To understand the mechanism of directionality, mutant integrases were characterized that were active in excision. A hyperactive integrase, Int E449K, gained the ability to catalyse attL x attR, attL x attL and attR x attR recombination whilst retaining the ability to recombine attP x attB. A catalytically defective derivative of this mutant, Int S12A, E449K, could form stable complexes with attP/attB, attL/attR, attL/attL and attR/attR under conditions where Int S12A only complexed with attP/attB. Further analysis of the Int E449K-attL/attR synaptic events revealed a preference for one of the two predicted synapse structures with different orientations of the attL/attR sites. Several amino acid substitutions conferring hyperactivity, including E449K, were localized to one face of a predicted coiled-coil motif in the C-terminal domain. This work shows that a motif in the C-terminal domain of ϕC31 integrase controls the formation of the synaptic interface in both integration and excision, possibly through a direct role in protein–protein interactions.

The Streptomyces phage phiC31 encodes an integrase belonging to the serine recombinase family of site-specific recombinases. The well studied serine recombinases, the resolvase/invertases, bring two recombination sites together in a synapse, and then catalyse a concerted four-strand staggered break in the DNA substrates whilst forming transient covalent attachments with the recessed 5' ends. Rotation of one pair of half sites by 180 degrees relative to the other pair occurs, to form the recombinant configuration followed by ligation of the DNA backbone. Here we address the nature of the recombination intermediates formed by phiC31 integrase when acting on its substrates attP and attB. We have identified intermediates containing integrase covalently attached to cleaved DNA substrates, attB or attP, by analysis of complexes in gels and after treatment of these complexes with proteinases. Using a catalytically inactive integrase mutant, S12A, the synaptic complexes containing integrase, attP and attB were identified. Furthermore, we have shown that attB mutants containing insertions or deletions are blocked in recombination at the stage of strand cleavage. Thus, there is a strict spacing requirement within attB, possibly for correct positioning of the catalytic serine relative to the scissile phosphate in the active site. Finally, using integrase S12A we confirmed the inability of attL and attR or other combinations of sites to form a stable synapse, indicating that the directionality of integrative recombination is determined at synapsis.